Not all parts of a topic belong in a community. For example, let's say I have a community about car mechanic advice. The relevant topics are probably #cars #auto_repair and #mechanics. However, #cars can also apply to new cars, deals on used cars, or the movie cars, none of which are directly relevant to auto repair. Likewise, #mechanics can apply to airplane mechanics or even video game mechanics. Trying to match communities to sets of hashtags is going to be noisy, so you'll get a lot of false positives and false negatives.

Likewise, not all individuals in a community are worth following, and individuals often post about different topics than the ones in a community. If you're interested in cars and I post about cars, you may want to follow me. But I may also post about cryptocurrencies and lawn care, and you may not care about those at all.

Trying to mix Reddit, Twitter, and Facebook style posts doesn't particularly work. It's better, IMO, to use services that do each of those well separately, and cross-post from one to another when you think it's relevant. Treat them as islands, and build bridges between them, don't try to mash them together into one SM soup.

The words in the middle are the key difference, though:

Communities are groups

Communities involve (i.e. are not) topics

Communities involve (i.e. are not) individuals

The social structure of Lemmy is fundamentally centred on groups, and that's what makes it distinct from other fediverse platforms, even if there is some interoperability