(Yes, that's a Button !) Ignore the Letters on the Joypad
The "A" Button in Recalbox is the "B" Button on the Joypad
The "B" Button in Recalbox is the "A" Button on the Joypad
The "X" Button in Recalbox is the "Y" Button on the Joypad
The "Y" Button in Recalbox is the "X" Button on the Joypad
You can see it here:
Look where the "dark Point" is in the small Picture on the left beside the Letters A, B, X, Y.
Buttons Start, L1, L2, R1 and R2 are the same as on the Joypad
For Button L3 you have to press (NOT PULL DOWN) on the left analog Stick, for R3 on the right analog Stick.
If you're not sure how it works try it bevor you start the Configuration
On the Back of the Joypad set the Switch to "X"
To avoid further Confusion with the Joypad-Buttons in the Main Menu of Recalbox got to
UI Setting -> Theme Configuration -> Theme Iconset and set it to "XBOX"
Now the Letters on the Joypad will match the Letters of the Buttons you use in the Menu of Recalbox (on the Bottom of the Screen)
This setting works fine for me since Years on my Raspberry Pi 3B+ with Recalbox 6 and any game on any System.
If you playing Games from a System which uses another Button-Layout (for example the Sega Genesis, which uses a 3-Button Pad
with the F310 Controller, you start the Game and simply push the Buttons to figure out which Button is A, which is B and which is C.
The same goes for other Systems like Nintendo.