Documentation has a hard time covering every single topic the engine handles. If google is not helping you and there’s isn’t already an answer in the forums or other community supported places. You could also ask around on the Unreal Source Discord, it is quite active.
You might also get some insight asking something like Chat GPT; I find it’s a good solution to going from “i don’t know what i don’t know” to “i have some idea of what i need”, which lets you refine your search. Sometimes you get lucky with Chat GPT and it actually gives you the correct answer you’re looking for.
With custom depth pass for example, it might be something the engine does but didn’t invent, as it is a more general graphics rendering/programming concept you can just learn about on its own.