Based on other existing material on the topic (like the excellent code_report youtube channel) I once wrote an introduction to combinators and lambda calculus targetted at javascript developers (mostly targetted at my younger self) [1]

In short a combinator is a pure function that accesses only identifiers that are provided as arguments.

Length(x,y) { sqrt(xx + yy) } is not a combinator because it relies on global definitions for plus, times and sqrt.

But foo(x, y, b, u, v) { v(b(u(x), u(y))) } is a combinator because it only composes functions that are given as arguments.

Foo(3,5,+,square,sqrt) would result in the same value as length(3,5) so foo can be regarded as capturing the compositional structure of the euclidean distance calculation.

[1]: https://static.laszlokorte.de/combinators/