How infer keyword works

/**
 * If the type `P` passed in is some kind of `PromiseLike<T>` 
 * (where `T` is a new type param), extract `T` and return it.
 * If `P` is not some subtype of `PromiseLike<any>`, pass the 
 * type `P` straight through and return it 
 */
type UnwrapPromise<P> = P extends PromiseLike<infer T> ? T : P;
 
type Test1 = UnwrapPromise<Promise<string>>
//    ^ type Test1 = string

type Test2 = UnwrapPromise<Promise<[string[], number[]]>>
//    ^ type Test2 = [string[], number[]]

type Test3 = UnwrapPromise<number>
//    ^ type Test3 = number