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
Favourite, not sure. Maybe my “favourite” would be the one which would be the hardest to replace with something I like.
There wouldn’t be something i can think off that could be irreplaceable. However the hardest thing I like may be FanControl.
For the browser, Firefox is very nice, but it’s “just” a browser if you think about it. There is brave, and other open source chromium alternatives if it disappears.
For mail clients, I also like the Mailspring design, however Thunderbird just got a new skin and damn it looks good too.
And for the rest, I don’t really know. Either I don’t remember right now, or no special “like” for the software. Or I like the closed source software convenience more (I may also have no idea of an open source alternative, or an equivalent in features open source).
It depends on the usage really.