Element/Matrix, well, it's kinda tough to say. I'm not sure if there's any good communities in there you can just get access to without knowing someone who can invite you in (which defeats the purpose), especially since less people use it sadly. I do know some general tips though. First off, choose your homeserver wisely, as depending on which homeserver you choose, certain things may happen like communities not letting you in, you being more censored/prone to getting your account yoinked, and possible restrictions e.g. on file uploads. Secondly, as far as I know, there is no disboard equivalent for matrix, so the closest thing to it is the "Discover" tab, which will just show you stuff from your homeserver by default, meaning you will have to manually input other servers to discover public servers on there. Thirdly, keep your encryption keys safe. Seriously. It will be a huge pain in the ass if you lose them and you will lose access to the ability to see your old messages, so store multiple copies and always have one copy at hand on whatever device you wish to be using. And last but not least, the default homeserver, matrix.org, is apparently quite shit, so you will have to do some digging around to find out what the good ones are (midov used to be one of them but they disabled file uploads a while back apparently, so it's less good now). I think that's all the info I can give you, since I haven't been in the know with matrix for a while