This earlier post seems to provide some useful context: https://writings.stephenwolfram.com/2020/12/combinators-and-...
* I is the identity function: Ix = x
* K is the constant function: Kxy = x
* S is a bit more complex: Sxyz = xz(yz)
Y = S(K(SII))(S(S(KS)K)(K(SII)))