You might find one of Lin’s talks good for a primer on WebAssembly: https://forum.dfinity.org/t/webassembly-resource-links/62/3
Rust and Motoko can both compile to WebAssembly and run on the Internet Computer, so you can choose either (even both) this is what Hans was referring to. But it’s worth keeping in mind that Motoko has features which make writing for the Internet Computer easier, including hiding some complexity so you can concentrate on other things.
Javascript is what you want for the front end, as you’re already used to, and you can call your WebAssembly canisters’ public methods from your javascript code, some of the tutorials you’ve looked at show this in action.