Sound engineering. Most people think sound is somehow sine waves and that's it. And well, that's technically correct on some level, but you can layer sound waves on top of each other to create triangle waves or square waves or what specific instruments happen to sound like: Source

And well, these aspects have implications. Like with an oboe, even the basic waveform is quite interesting, so it's excellent for solos.
On the other hand, with a more boring sound, like a sine wave, you can do relatively wild things in terms of melody or combining them into intervals, and listeners won't feel overwhelmed as quickly.

And then you've got the fun field of drums. You can often just take white noise (or pink noise etc.) and just make its volume drop off rapidly and that already sounds similar to a drum.
Which is again interesting on the boring/interesting spectrum. That noise signal adds a short moment of chaos into the mix. But then we often make drums play quite structured rhythms to entertain a different boring/interesting spectrum over time.

Linguistics. Did you know English and Bengali are related? They share an ancestor about 5000 years ago. Russian, Latin, Farsi, and Greek, and lots more are in that family too.

Do you know what languages are not related at all, absolutely 0% aside from borrowings, even though if you know a bit about em they seem like they should be? Japanese and Korean.

Connections.

I love the stories about how seemingly disconnected events at different times and in different parts of the world actually have a connection. Like how some random chance meeting of two people 100's of years ago started a chain reaction that ended up with us having some cool new technology or idea.

FPGAs, love the damn things. They're circuits that you can re-program at will after they've been manufactured! If you build, like, a 2-input AND gate, that's all it will ever be. It can only take in 2 inputs and AND them together. But with an FPGA, they're manufactured to be versatile; you "program" the circuit you want to achieve onto the chip, and it will achieve that functionality! You can make a 2-input AND gate, slap it onto a bread board, and have yourself that nifty little AND gate, but if you later decide you wanted it to be a NAND gate, just reprogram the chip and like magic, what was once an AND gate is now a NAND gate. They're great!

Theology. I've read so many religious texts from all sorts of religions and while many people might discuss the organizations of religion a lot, or make fun of religious people, I rarely get to talk about the belief systems and their cultural relevance to various peoples both now and in the past or even discuss the possibilities of God and what God may be if one existed. I don't study for the belief itself, I personally am an atheist; but knowing these belief systems helps understand people a bit better. Plus some of them are actually full of kick ass stories. Hindu is insane with space battles and shit.

I literally cannot answer this without seeming like a pretentious, arrogant ass. Anyway, quantum field theory, high energy physics, condensed matter physics, generative art, computing algorithms, arcane math theories (meaning difficult to understand, not magical), procedural art, simulations, awesome places to visit in the world, Factorio, the channels I watch on YouTube. Honestly, I don't have anyone around me who cares about or understands most of my hobbies. They all love me and care for me, just not the stuff I like.

Cassette tapes. They may not have the most accurate sound, but they have a cool, unique sound, and they feel really nice to hold. People who love vinyl often hate tapes. I love both.

Computing history, obscure operating systems, movies and music my friends never heard about, fringe humour nobody gets.

I should probably add something of my own. I like failed firearms designs that had some features that makes them objectively better if you overlook all the downsides. Or generally anything where one person made it because they thought it was cool and no one with better judgement stopped them.

Like the Gyrojet pistols, unlike traditional pistols they used rocket projectiles. This meant two major things one the guns didn't need to have any pressure bearing parts and could be made incredibly light compared to normal pistols (.88lb/.4kg vs ~2.2lbs/~1kg), and the down side they reached maximum speed much slower than normal cartridges.

This meant that in practical terms a target close enough might not give the rocket sufficient time to accelerate to effective speed and wind would have a greater effect on the projectile. Hence they don't make them anymore.

Chess! I suck ass at it so I don't fit in with actual chess players, and everyone I know is totally disinterested in the game. I can only fuel my love for the game by watching Gotham trash talk 1200 rated players when I know that I'm a lichess 700

Astronomy! Not astrology! No, that's not a smudge on the lens, that's M3, and it took me an hour to find it in this Bortle class 7 suburb with my 4.5" dob, and I'm damn proud of myself for that

Anime! No, I haven't seen Naruto, or DBZ, and I ain't got time for One Piece. How's about Shinsekai Yori, Haibanei Renmei, or ACCA? Nah, nobody I know has seen those, even though we're in our late 20s and are a bit old for shounen at this point

Morrowind.
True conspiracies. Cryonics.

Asian beauty items, specifically kbeauty and jbeauty skincare. I love hearing about people's skincare routines and new niche items, ingredients and brands. But my friends irl aren't really into beauty (everyone i know is so pragmatic lol).

I really love beauty empties- I used to follow subreddits related to finishing beauty products, aka panning them (panning meaning hitting the metal pan at the base of powder cosmetics; panning has come to generally mean using up your products). There is something so satisfying about using up your stuff and I like vicariously experiencing it through other people. I mentioned that I was saving my empties for a few months to review which ones were worth repurchasing and my friends laughed at me, thinking I was joking.

Obscure 80s and 90s hip hop records that very few people have heard of because they sold less than a couple hundred thousand records, some much less

Classic black and white films and TV shows. No one I know in real life enjoys these.

The State Emergency Services of Australian states. Basically, they're state government funded volunteer rescue agencies that focus on floods and storms. They also provide other rescue services outside of major population centres, like vertical rescue and road crash rescue. I think it's a great idea because we get enough severe storms and floods that a specialist agency is worthwhile, but there can be years between large scale events, so having the surge capacity of a volunteer agency is great.

It's a similar story with the CFA/CFS/RFS/NTFRS¹/TFS¹/whatever's happening in WA¹, but instead of rescue they do firefighting. Their main ("combat") role is fighting bushfires, which happen seasonally so surge capacity is important. However, in country areas they also provide structural and vehicle firefighting services. I'm aware that the US have a similar situation with volunteer firefighters though, so I'm not so keen to teach grandma to suck eggs.

¹These agencies are metro and rural firefighting agencies, so their combat role is all fires, not just bushfires.

Googology. The "study" of ridiculously large numbers. It's a rabbit hole of recursion and mathematics that starts with stuff like googolplex (10^10^100) and never really ends.

Rhythm games, 100%. I am a fair bit obsessed with them but I have yet to find a friend who truly Gets It. They either only like music, or they like games. Somehow never the marriage of the two.

I feel like people could benefit from what I know about Butt Stuff™.

NA and NULL values.

In R (programming language) they have some interesting differences. You can think of a vector as a train with many cars, and each can hold a number. Let’s say I have train with three cars and I store the number 2, 3 and 5 in them. That would be a normal well behaved vector (2, 3, 5).

I could take away one of those numbers and leave that seat vacant. It could look like this (2, NA, 5).

If I tell you to find the third number in that vector, that’s easy. It’s 5. If I tell you to find the ninth one, that just doesn’t make sense and the answer would be NULL.

So in other words, NA is a vacant seat with no number sitting in it. NULL is a place where there is no seat to begin with.