‘Configurations by core’ doesn’t exist anymore. It’s been superseded by the override system.
The way the controls work is like this: you have a physical pad that you hold in your hands and then there’s a virtual gamepad abstraction called the ‘retropad’. The retropad looks sort of like a wii u pro controller and we provide a default mapping of the core’s inputs onto that retropad abstraction. When you go to settings > input >p1 input binds, you’re mapping the retropad’s buttons onto those of your physical pad. Then, when you load a core and some content and go back into the ‘quick menu’, there’s a menu named ‘controls’ where you can move the core’s inputs around on the retropad (i.e., you can change the default mapping we provide).
If you’re using a gamepad that already is autodetected and autoconfigured by RetroArch, you can just go straight to the ‘controls’ menu and it will list your pad’s buttons by name and you can move the core’s buttons around accordingly.