How do I turn on sharing for ultimate

Yes I know but we are on the same Xbox, the only way I can play online or play the game he downloads with ultimate is if I log into his account

He would have to set the console as his Home in order to share with you, but keep in mind things like perks are only available to the account that purchases the subscription. You'll be able to play online and access games he downloads. https://support.xbox.com/en-US/help/hardware-network/console/my-home-xbox

If you do the ultimate you can play on PC too but I don't know how sharing works

How would one do so if they are on pc but have gamepass ultimate and are trying to share that with a separate xbox account on an xbox.

Your account won't show as owning Game Pass Ultimate since it's only being shared. And Game Pass itself doesn't share, only the games downloaded from it do. So if the account that owns Ultimate downloads a game, you should have access to it.

If you haven't tried already though, try a hard reset (hold down the power button on the console until it turns off, unplug the console, plug it back in, then turn it back on).