Servers do not "follow" servers. Servers may allow federation with other servers (utilizing either white or black listing based on specific configuration) but do not make requests to a server unless they have a user who has followed a user on that server.
The word "user" here can refer to several different types of entities including your user account, a peertube channel or a lemmy community.
If you follow a peertube channel then it should show up in your timeline provided that your filter settings are not excluding it, say for instance by viewing the local only timeline.
This is my layman understanding at least from social osmosis but I have never read the activity pub specs.