On Mastodon it's pretty easy. Download the official app and go through the prompts. They should probably have a little note saying "just go with the defaults if you're not sure" but this shouldn't be a road block for any normal person. The fact that Mastodon has a standard migration method makes this a low-impact decision.

Lemmy is definitely harder. "Jerboa" doesn't sound like an official app, and I don't think you can even create an account in Jerboa. So the first step is finding an instance on the web with no guidance. That's bad.

I still haven't joined Matrix because it's too hard. People say I shouldn't use matrix.org for various reasons (like bans without warning) but I can't find an alternative that seems sensible. All the guides I found are basically "you should really host your own, but if you're too much of a noob, here are some Polish lolicon-themed servers you can join". If it were possible to sign up without feeling like I'm doing something wrong, I would have many years ago.