The developers are basically anyone who wants to get in GitHub and help add to, or alter, the source. There's some of us who've did it enough, we've kind of been recognized as "official" developers, but anyone can help with the project. Find something that's wrong, and you can fix it? Join the GitHub and push the change. Need a feature, and know how to add it? Join the GitHub and push the change.
That's basically all it takes to develop QB64. ;)
AS for the features and future, those are honestly, more or less, up to each individual contributor. I needed data compression, so I added in the ZLIB compression routines with _INFLATE and _DEFLATE. Kobalt needed routines to toggle and work with bits easier, so he added in _SETBIT and such. It's basically been a case of "if you need it incredibly bad, and you know how to implement it, then feel free to do so"! As far as I know, there's nobody setting any sort of master plan as to where the project should go in the future. There's no "let's swap out glut and move to XYZ instead in the next year" type discussions that I'm aware of. If someone wants to do such, they can always feel free to branch off from the main repo, do their thing, and then if what they've accomplished works and is proven better than what we currently have, their changes can be merged into the main branch.
As far as keywords and syntax are concerned, those are often discussed in Discord, before being implemented, to keep them "in style" with the language, and to make them something everyone can agree upon. Me, personally, I tend to not care that much, so I'd be happy to add in commands like _DFLT to reduce typing, but everyone else prefers something more verbose and in the spirit of BASIC, like _DEFLATE. Sometimes, someone will add a command where the syntax doesn't seem just "right" to everyone else, and then, since this basically an open-source project, somebody else will tweak it while it's still in the development build and change it to something more suitable. And, in the case that the two people end up fighting back and forth, and they can't compromise, at the end of the day, the final decision rests in the hands of those who maintain the repo for us -- Fellippe and Luke. (And maybe Stx? I know he helps maintain the forums here, but I'm not certain if he does the repo itself, or not.)
Donations, as far as I know, go directly for server upkeep and such things. Nobody is making any money off the project -- it's all a labor of love. ;)