When a user uploads an image or video on Mastodon instance 1, and a user of Mastodon instance 2 is following them, that image or video is copied over to Mastodon instance 2 - because that’s where that user resides.
The same thing happens with text posts, right? I don’t see an exponential expansion, just linear in the number of nodes. It sounds like the decentralized way to do things. Hmm. Anyway, thanks for the explanation. It saves some storage but doesn’t save bandwidth, it sounds like. Rather, the bandwidth requirement gets concentrated at the shared server.
I wouldn’t bother with the Fiverr thing but interesting personal projects and FOSS contributions are both good. Sizeable FOSS projects mean you’re working with other people which brings both benefits and challenges, and more closely resembles the “job” world. You could also look for actual paying work (not gig work like Fiverr, that is crap) if you have the time for it (summer job might be possible). Look at the monthly “Who is hiring” thread (first weekday of each month) on news.ycombinator.com, look on craigslist, etc.
Getting involved in FOSS is pretty simple. Find a project with a list of open tasks or an issue tracker, find something that interests you, say you are interested in working on that task, and start contributing patches. Usually if the project is not a high-visibility one with a lot of contributors already, it will welcome any help it can get. Lots of such projects have Freenode IRC channels where you can chat with the other devs in real time. I’m less comfortable with the ones that use Discord, but that’s just me.