Monkeytype is open source (code on GitHub), but uses Firebase for hosting and the whole source is really tied to it as far as I undertand. Also they use Discord for everything, going from quote submission to feature requests and bug reports. GitHub is also used but I’ve always felt like the dev’s attention is mostly the Discord.

Otherwise it’s pretty great and the most feature-full of them all IMO. Literally everything is customizable and there’s lots of modes to play around with. I’ve been using it to learn Colemak-DH the past months and it’s been really fun.