how to alter the keyboard layout?

Hi,

how do I alter the keyboard layout for plasma on wayland sessions?

I need to put Alt on the left menu key, a dot on the Delete key on
the number pad (instead of a comma) and a slash on the '+' key
(wich is right to the 'ü' key)?  I used to do that with an xmodmap,
but of course that doesn't work anymore with wayland.

There must be files somewhere that define all the various keyboard
layouts you can pick from, but where are they?  That kind of
information must not be hidden from the users and should be shown in
the settings ...