Hello. Thanks for checking my game out. Mostly, it is made from scratch. I do, however, always steal bits from tutorials here and there. It's just the smart way to work. I've been programming for so long now, I often use tutorials for one engine and just make it work in another engine. The principals are usually the same.
Anyhow, for this game, unfortunately, I can't point you to much. To learn Godot (after learning a few other game engines first), I just used the standard "Dodge the Creeps" tutorial here: https://docs.godotengine.org/en/stable/getting_started/first_2d_game/index.html
I did spend some time looking at the Lua code in this "Succer" game: https://www.lexaloffle.com/bbs/?tid=2614 . (Pico-8 games have their Lua code embedded in the "cartridge" PNG files.) It helped a bit, but I can't say my code or game structure looks anything like that. Pico-8 is a very different animal to write for -- it's more about keeping it minimal rather than writing smart.
Mostly, however, Sendit Soccer is about developing new browser-based multiplayer tech, and that is all me. I've used some tutorials to help that along as well... but that's getting way beyond just learning Godot. Anyway, I hope to get back to this game in a few months to make it much more interesting (but gots to earn some cash first). It appears to be my most popular game yet and deserves more attention from me (not to mention what else I might build with this web tech once I make it more robust).