It is actually very easy, on Gnome you need to open nautilus(Files) then right click the corresponding file type, click "Open with..." and select what app you want it to open with and if you would like to make it the default. Which funny enough is done the same way in MacOS. You just overlooked it.
PS:This method automatically writes changes to ~/.config/mimeapps.list
PS2: Usually you can find answers to this questions in the documentation with a quick online search.
maybe this can help