Mastodon and others are FOSS, yes. People either volunteer their hours or are paid by an interested party/company to working on FOSS projects, but in both cases they release their work for free to everyone. Even if they start corrupting their codebase to appease corporate sponsors, the community can fork the project and keep it going without the original developers’ influence. E.g. it doesn’t matter if Eugen gets a railroad spike through the head and decides that putting ads on Mastodon is a good idea, we will fork the project and continue on without them.