How to add a directory to your PATH

I was talking to a friend about how to add a directory to your PATH today. It’s something that feels “obvious” to me since I’ve been using the terminal for a long time, but when I searched for instructions for how to do it, I actually couldn’t find something that explained all of the steps – a lot of them just said “add this to ~/.bashrc”, but what if you’re not using bash? What if your bash config is actually in a different file? And how are you supposed to figure out which directory to add any