Hello! Came here from Reddit. How can I "follow"/"join" this "sub" like there

You should be able to just by clicking the button in the sidebar.
On mobile, you might have to unhide the sidebar first.
Or go to lemm.ee/communities?listingType=All and search for communities that interest you. There should be a subscribe button next to each one.

Other that that, it depends on the app you use to access lemmy.
In my opinion, just the web interface in a browser is good, on mobile too.