I get why people like to split code in multiple files for maintainability but its a fucking nightmare if you want to understand code and you have to go from file to file to file to find the origin of 1 API-Call.

Well this is where a good IDE becomes useful. Going to declarations, definitions, usages etc. is extremely handy. Any kind of sizeable project will have multiple files. Hell, it'd want to. I don't want to have to scroll through hundreds of lines of function definitions that I don't necessarily care about at the time. Best that they're compartmentalised in a nice tidy fashion.