Quite the opposite. Use drives from as many different manufacturers as you can, especially when buying them at the same time. You want to avoid similar lifecycles and similar potential fabrication defects as much as possible, because those things increase the likelihood that they will fall close to each other - particularly with the stress of rebuilding the first one that failed.

You can use different manufacturers, just make sure they are the SAME size and speed. You can also get the same ones from the same vendor, just from different online shops to try and offset getting a bad batch.

I always thought you're supposed to buy similar drives so the performance is better for some reason (I guess the same logic as when picking RAM?) but this thread is changing my mind, I guess it doesn't matter after all👀

You absolutely can. Of course you'll only be able to use as much capacity as the smallest disk. Sometime ago I was running a secondary mirror with one 8TB disk and 3 disks pretending to be the other 8TB disk. They were 4TB, 3TB and a 1TB - trivial with LVM. Worked without a hitch for a few years till I replaced the three gnomes in a trench coat with another 8TB disk. Obviously that's suboptimal but it works fine under certain loads.

Hardware or software (BTFRS, ZFS etc...) RAID?

As long as they're mostly the same. For example on many controllers no mixing SSDs with HDDs.

If you haven't looked into it, and if you already have the disks of varying capacity, check out JBOD. You will have to configure a system for backups however as you wont have parity like raid1

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

3 acronyms in this thread; the most compressed thread commented on today has 15 acronyms.

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I usually find the cheapest drives and buy multiple of those, but you should be able to assemble a RAID out of different disks, though you'll be limited to the space of the smallest one in the mirror set.

Also make sure that your RAID systems supports this.

I would strongly recommend that you get the same drive. It doesn't make any sense not too