Your instance knows

• Your IP address
• Your E-Mail (if provided
• Your password (salted of course)
• Your session type (browser, app, etc)
• Your interactions (posts, comments, saved, upvotes, downvotes, etc
• Your social graph (following and followers, etc)

Everyone in public knows

• Your session type (optional)
• Your interactions (posts (if set public), comments, saved (depending on service), upvotes, downvotes, boosts, etc)
• Your social graph (may be optional depending on service)

Nope. It's public. Assume everybody has a copy of everything sent over ActivityPub.

Only your instance host could monitor your personal metrics and fingerprinting. These are what are so invasive with big tech.

Everything you post here as content is public and can be scraped you should assume everything will be mined for data.

They can't see your IP, dwell time, votes, what you read, what you didn't read, and most importantly they can't inject content into a structured echo chamber and observe how you react.