Why Metada=() in Pointee<Metadata = ()> can be passed to Pointee Trait?

For a detailed description of the problem, please check this link

It's just a normal associated type bound, just like

fn take_iter<T>(iter: T)
where
    T: Iterator<Item = u64>,

which constrains the associated type to be the given type. In the case of Thin it's constraining Pointee::Metadata to be the unit type ().

I'm not really clear on what you're asking when you say "how it's passed"

I still don't understand how Metadata = () is passed to the Pointee trait, which does not contain the generic

Ahh, maybe you're confusing type parameters with associated types. When you pass a type parameter you don't use the Name = syntax, that's exclusively for associated types.

For example the Into trait has a type parameters specified by Into<u64>.

So Pointee<Metadata = ()> is constraining the type Metadata inside Pointee's declaration

Do you have any recommendations for information about type parameters and associated types?

The rust book has a section on associated types

https://doc.rust-lang.org/stable/book/ch19-03-advanced-traits.html

Type parameters in traits work mostly the same way they do on structs and enums