I bought the gamepass ultimate on my xbox and there's the discord nitro perk but when i look for it on the xbox i don't have the gamepass ultimate
It's not on the Xbox. You'll need to download the Xbox app on a PC. You can click on "Advantages" or "Perks" somewhere. You scroll down there if necessary and there's your nitro.
Believe it is also available on the mobile app.
I mean on the console not on the cell phone or pc
But you can only talk on a cell phone or PC.
Wouldn't make sense otherwise. There's no Discord on Xbox, only on a cell phone or PC.