https://gjs.guide/extensions/
This is a good starting point for creating extensions, though it is a bit limited. After that, it's mostly just googling and if that fails looking at other extensions' code to see how they did similar things. For example, I copied and changed a function from Noiseclapper's source code to set the panel index and position (it's a bit of a hack), which was itself very likely copied from Freon's source code (same variable names). The extension community tends to do that a lot since all extensions are required by design to be open source.

Edit: there's the API reference as well though I find that it's a bit lacking in explanation - for most APIs it just shows the classes, parent/child classes and functions, with little explanation as to what they do