Core supports sub-folders started with _ character.
Regardless the place of RBF file, boot rom/vhd should be placed into either root of SD card or core's dedicated folder (should be created in root of SD card).
Joystick (including emulation by keyboard) buttons defined in this core is default map for all cores unless defined in particalar core.
Wallpaper
Place menu.png or menu.jpg to the root of SD card to have it as background on HDMI (you can use vga_scaler=1 if you want it on VGA).
Create "wallpapers" folder and place multiple .jpg or .png to it. Use F1 to cycle between standard MiSTer backgrounds and wallpapers in folder.