Layer Modes, sometimes called Blending Modes, allow you to combine layers in a variety of ways. We'll begin with the Normal modes and work our way through many, but not all, of the Layer Modes available on the latest (at the time I write this) version of GIMP, 2.10.20.

Links:

https://docs.gimp.org/2.10/en/gimp-concepts-layer-modes.html

https://www.ahuka.com/gimp/normal-layer-modes-normal-dissolve-color-erase/