Default "Hotkey" button mappings for RetroPie can be found here: https://github.com/retropie/retropie-setup/wiki/RetroArch-Configuration#default-joypad-hotkeys
Hotkeys Action
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Select Hotkey
Select+Start Exit
Select+Right Shoulder Save
Select+Left Shoulder Load
Select+Right Input State Slot Increase
Select+Left Input State Slot Decrease
Select+X RGUI Menu
Select+B Reset