All they need to do is write #myhobby in the search field whatever their hobby is. On the web interface, there is then a person with a plus sign on. Click on that and you are following that hashtag and will see all posts tagged with it in your home feed.
While you write the word(s) behind the #, the web interface even lists matches to the word and longer hashtags beginning with that word - and they tell you how many toots it gets a week.
@MaryPot @flit @human3500 @Mastodon Except that only works if #people #post like "Really enjoyed my #badminton training today!" and if you post like that, you look like a #twat.