And your example helps me illustrate:

How does your client know who you subscribed to?

1) You wrote a list "follows" or "favorites" or "people who know about Formula-1 racing" and posted an event containing the list, and the client lets you select that list.

2) Your client lets you select someone else's "Cool Nostriches" list.

3) You client assumes you subscribed to everyone not on your "mute" list.

4) You paid to be let onto a relay, that you know lots of interesting people write to, and the client allows you to add that relay and see the notes in your feed.

5) You plugged into a DVM, that suggests interesting notes to you in your DMs, and your client doesn't block those DMs.

6) You downloaded the source for a client (like oddbeans) and changed the filter code, to show you what you want to see.

Etc. There are many different algorithms for showing you the notes you want to see.