Changing mouse and keyboard key

What would be the best way to change keyboard and mouse key in gnome. I’d like to switch the fn and strg either on the internal or external keyboard to have it the same on both of them. I’m not sure if I can do that only for one keyboard and not the other.

Also I have a mouse with more buttons I can currently use. I’d love to have two of them mapped to strg+alt+left/right to switch desktops?

Thanks for any hints.