Logitech Mutli-Device K780

switch slots

press one of F1,F2,F3

connect to a new device

press one of the keys metioned in switch slots for 3 seconds

change keyboard layout to Mac

press fn + O for 3 seconds

change keyboard layout to Windows

press fn + P for 3 seconds

swap function keys | swap fn keys

Arch Linux

sudo pacman -S solaar

sudo solaar show

Hint: Device numbers in solaar show do not necesarrily reflect device numbers used for solaar config

# device number in `solaar show was '2'`, number I had to use was '`3`'
sudo solaar config 3 fn-swap False

Mac OS | Darwin

$ defaults write -g com.apple.keyboard.fnState 1
$ defaults read  | grep keyboard.fnState
        "com.apple.keyboard.fnState" = 1;