I can only see new posts. It doesn’t retrieve older posts

This is normal. You'll only receive new posts going forward from ActivityPub. Think of it like joining a mailing list, you'll only receive new mails after subscribing. If you want to pull older post, you'll have to do it manually by pasting the post (or comment) url in the search field.

You don't get federated automatically. On your own instance you will only see posts in communities that you subscribe to. What I did was set up a "bot" user that I used to subscribe to a bunch of communities that I might find interesting when just browsing the All feed but I didn't want in my personal Subscribed feed. There's a few tools that will automatically subscribe a bot user on your instance to all the top communities but I found just manually going through https://lemmyverse.net/communities and clicking subscribe as that bot user worked for me.

I set up this: https://github.com/Fmstrat/lcs

It goes somewhat slowly but it's been pulling in communities for me.

What version are you using? There were issues federating with v 0.18.0. I updated my instance to 0.18.0-rc.10 and that resolved the issues.

Things don’t get backfilled, so until a new action happens on an old post/comment/etc they won’t show up on your instance. New things should make their way in eventually though.

Taking the link of a specific post/comment from the community instance and searching for it from your instance should populate it on your instance, just like you probably had to do to get this community to show up so you could subscribe/post at all.

There are backfill tools/scripts, but unless you really want old posts I wouldn't use them. It unnecessarily increases the load on already struggling popular/overloaded instances like lemmy.world.

For micro/personal server runners, I built a tool to automatically discover and add communities to your local instance :)

https://github.com/lflare/lemmy-subscriber-bot