Sorry if this is a dumb but I legit never got into Twitter, and I only use Instagram to follow friends and bands I like.
How do I Mastadon? I'm not being sarcastic, not even a little. Like I literally have absolutely no concept of what I'm supposed to do on it or how to engage with it. Same with pixelfed tbh, like I open it, I see a milliong posts that have no comments or likes, I get confused and then I leave.
Like what do you do? How do you use it? Pretend I'm one of the idiot journalists this post is making fun of, happy to jump on that self-accepting sword!
It's just a different way to browse current topics that people are discussing. You can follow famous/not famous people, news people, musicians, artists, scientists and so on. You have to take some time to search by name or a hashtag like #music that is interesting for you and then follow those. They typically lead to more people and hashtags of interest that you can follow to build a more personal feed. It's just a different way to curate the various things that interest you.
The thing is that it's just another option for people to interact like lemmy/reddit twitter/mastodon pixelfed/facebook etc. Obviously the less popular options have less niche interests. Journalists see that these options can't be used the same way, and need some work to figure out and navigate, so they critique the different and less polished things they see. If they don't have what you are looking for, maybe check back in 3-6 months when there are more users and activities. Like lemmy, things are changing quickly right now.
"Searching for a subject matter that interests you" in Mastodon/PixelFed is all about hashtags. If you're interested in science fiction, try #scifi. You would do well to either sign up for an instance relating to your interests (for example a dedicated scifi instance, like what startrek.website is for Lemmy), or a general purpose one (mastodon.social) where you can get a better overview. If you find an instance related to your interests later on, consider moving there.
As you search hashtags, you'll find interesting people. Follow them. You'll see some interesting posts. Boost them. Adding a star to a post serves no practical function, but it'll make the poster happy anyway (it's the same as upvotes here).
Mastodon is based a lot around boosts - if you see someone boosting a lot of content of the type you're interested in seeing, make sure to follow them and your feed will be populated by content curated by humans, not algorithms.
As long as everybody are federated - but they are not. The bigger instances tend to be federated with everyone, but the smaller/more specialized ones might not yet have federated with each other. Of course you can still follow people from anywhere (as long as they're not actively _de_federated), but searching for hashtags might unfortunately be less powerful for discovery from a smaller server. If you're on a smaller instance not specific to your interests, you might do particularly well to start out finding accounts to follow through some third party list.
The greatest advantage of being on a specialized instance is, however, in the feeds: The "local" timeline shows you anything posted by anyone on your instance, while the "federated" timeline shows you anything posted by accounts someone on your server follows. On a large, general server both these feeds will be extremely crowded with all kinds of content. On a specialized instance, it's likely that both timelines will be somewhat interesting, and people might be reading it and discover your posts there as well. :)