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If you press '?' in the magit window a popup of available commads should appear. If I remember correctly from when I used to use DOOM emacs before switching to vanilla, evil (vi emulation) keybingings mean the keys listed in the magit popup are incorrect, however it should give you an idea of what commands are available and you can use other Emacs help facilities to find the Spacemacs keybindings.

The best UI is for a cheat sheet to pop up after a delay, so you can proceed without a delay if you know, but also not have to press anything else if you don't know, just wait a bit.

Does https://github.com/justbur/emacs-which-key not work with spacemacs?

try the ? key