The way I sussed out Mastodon was to go to the browser version of mastodon, find Settings, and enable "advanced web interface".
Then you should see columns. (a bit like twitter's tweetdeck)
Your home feed shows content from all your follows, and saved hashtag searches.
The local feed shows everything from people in your server
The Federated feed shows everything, everywhere on Mastodon, posted by Friends of your friends, or something like that.
If you search for a hashtag, e.g. #birds, then save this, you'll get everything with that hashtag in your home feed.
So your choice of Server is important but not essential. A Birder / Nature Server with lots of members would have a thriving local feed. I've found an Ecology Server that has useful stuff being posted in it.
The other useful thing I found was that if you find the three horizontal lines "Getting started" menu option, there is small print at the bottom. Click on "about Server", and look at "Moderated Servers". You'll see a list of other Mastodon Servers that the admin of your home server has banned so you won't see anything from anyone on these servers. The Server I am on seems to be banning any Servers with "Hate Speech", "Inappropriate", "Spam", "Misinformation", "Bots", so the feeds are subsequently very focussed in what I am interested in, and not much else.
The Birdon server above seems to have witheld the moderated server info, so I'd be reluctant to move onto it just yet, but would move on if it proves to be clamping down on conspiracy / hateful content, and starts getting more members.
I looked at both Twitter and Mastodon.. Mastodon is focussed on things I am interested in, very little in there that i'm scrolling past;
Twitter has a lot more stuff I'm not intrested in showing up, but does have more accounts from official bodies and rare bird alerts etc, so has more of everything, good and bad. I don't think you can just move off twitter and onto Mastodon just yet, maybe never(?).
