The issue with mastodon communities is that they don't use the same type of fediverse tools. You could maybe follow people from mastodon, but the apps or Lemmy/kbin will need to implement a compatibility with those fediverse websites.

To follow communities outside your instance it's a bit less optimised currently as the instances don't scrape all the communities from other servers.

So to find them, either someone else had to search it and subscribe to the community for the server to show it when you search for them, or you'll have to show the server where the community is :

For example a community from your instance : !apolloapp@reddthat.com

You can notice that I put a ! Then [community] and @[server address].

You will often see this presentation under the community name when clicking on a community : !apolloapp This is what that means.

If you were on another instance, to find the apolloapp community from reddthat.com, you'll need to search for either : !apolloapp@reddthat.com Or https://reddthat.com/c/apolloapp

I don't know if apps are able to search like this, so you may need to go on your account in a browser and search from there.

Also as stated in another comment, there are websites to search for communities, like Lemmyverse.

However you can find communities to replace the reddit ones here :

https://www.quippd.com/writing/2023/06/15/unofficial-subreddit-migration-list-lemmy-kbin-etc.html

And

https://sub.rehab/