You can post an activity somewhere (for example a photo on PixelFed or video on PeerTube), share it on Mastodon. You get a comment about this activity somewhere else. And you can reply to this comment from another place.
By the way, you should really consider trying Pixelfed. It’s a really nice community and Pixelfed is a great piece of software. No ads, no “algorithm”, no NFT. No bullshit.
Same for the Fosstodon community. Not too small, not too big. People are really great. Admins are awesome (this is why I already decided to help them financially). Thank you guys ! You are like friends.
With the Fediverse it’s easy to build a cozy community around a topic (of course communities are connected, they are not islands). A community is moderated by humans, and not strangers. You can discuss with them. It’s even really easy to contact the developers, for example Gargron or dansup. They are really nice. Thank you !