It depends what you mean by "understand" I've implemented a purely functional programming language that macro expanded in lambda calculus and was transformed i

It depends what you mean by "understand" I've implemented a purely functional programming language that macro expanded in lambda calculus and was transformed into various set of combinators for evaluation. So I know you can express the Y-combinator in terms of S and K to implement recursion and I've literally watched the resulting expressions resolve but do I really do not understand conceptually how you can implement recursion in terms of two ridiculously simple functions.