How HKT is actually working in fp-ts?

[SHND]

Check out this link from typescript playground. I gathered parts of the fp-ts in one place to test and understand how HKT works and simulated. It compiles ok, and runs ok.

I change the HKT1 and HKT2 to something totally nonsense from

type HKT1<URI, A> = { URI: URI; a: A };
type HKT2<URI, A, B> = { URI: URI; a: A; b: B };

to

type HKT1<URI, A> = { What: string }
type HKT2<URI, A, B> = { Hello: number }

But the code compiles ok. Why?!

The more strange part is if we comment the line...

function lift<F>(F: Functor<F>): <A, B>(f: (a: A) => B) => (fa: HKT1<F, A>) => HKT1<F, B>

...which is an overload and the default case. the code stops compiling!

How does HKT works here? How the types HKT1 is glueing Functor1 and Functor2 together under Functor?