@falktx Mastodon fetch posts from other instances using followed accounts on your instance. A post will be caught by your instance if:

it is from an account you follow
it is from an account someone else on your instance follows
it is boosted by an account you follow
it is boosted by an account someone else on your instance follows

Hashtags feeds on an instance list only those fetched messages, not more.

Mono-user instances can feel pretty empty because they do not catch as much posts as bigger ones. People tried to solve this by creating follow bots but nowadays, they are a bit frowned upon as some people used them to scrap content and harrass others