Option<T> is just a normal enum in Rust. Enums can also can have parameters in Rust. Those are usually called tagged union in non-rust-speak. It's either Some(x) or None. It has a pretty easy definition: https://doc.rust-lang.org/std/option/enum.Option.html

Storing None for an Option<&T> as a null and Some(ref) just as the reference is an special optimisation that the rust compiler does. Usually you have a selector which tells you which variant of the enum it is.