I joined discuss.online but is that the same as lemmy?

yes that's a part of Lemmy, that's all you need

I don’t understand the fediverse at all. The instances are confusing.

It's kinda like email. Think of it like you're on Gmail, I'm on Yahoo Mail, and the pottery community is on MSN. We can all talk to each other because we do like @mangobanana@gmail.com (in reality you're at discuss.online) and @die4ever@yahoo.com (I'm at retrolemmy.com). Usernames start with an @ like Twitter, community names start with an !

So when you do a "Local" search you only see stuff from "Gmail" like yourself, but usually you want to search "All" not "Local".

For viewing posts on my feed I prefer "Subscribed", this can be set as your default in your settings page.

have you tried an app on your phone? they can be a bit simpler to use than the website https://join-lemmy.org/apps

here's another community that might help !newtolemmy@lemmy.ca