Luau is not an official coding language, Lua is.
Lua has documentation at lua.org. It includes built-in functions like iterators and built-in fundamentals like functions or loops. It’s probably the best place to learn Lua if you aren’t talking about Roblox.
Don’t watch alvin or dev king. They prey on people who want to learn so badly and get them to keep watching their videos by making videos on how much developers make and stuff. Along with that their videos do not cover a lot and are not explained well/ enough for someone to learn. Especially the dev king, all he does is go to the dev hub and just read out what the article says. I just recommend learning your self by looking at api and the dev forum community tutorials. That is how I learned and I learned so much quicker than I did by watching kids on youtube. If you do want a half decent youtuber, yellow mustangs is pretty good but he quit and deleted his discord server idk why.
Both sites are good starts/places to learn lua.
I recommend you get some previous programming knowledge before trying out Roblox Lua. As others have said, try developer.roblox.com and try their tutorials, but you should also learn some programming before hand, so you would somewhat know what they might be talking about.
I think youtube tutorials and the dev hub are good for learning luau. I use AlvinBlox & TheDevKing mostly, with some others sometimes if some don’t teach me enough or miss a few stuff.