Can I play different games on xbox series x and xbox one at the same time

I was under the understanding that when you download an Xbox series x game it has different code, so it wouldn't be possible on xbox one x. Which would also be the same for the hard drive. The xbox series x game is apparently different than the same game
for the series x.

It actually would. There's an Xbox One, One S, and One X that are all in the same family. Then there is the Series S and Series X that are in a completely different family of consoles.

An Xbox One X is not the same as a Series X. The game will not work on an One X.

Hello there,

every Xbox 360 and or Xbox one game you can play via backwards compatibility on your series X/S share the same multiplayer network, so yes you can play those games with your friends across different generations of Xbox. However for future games releasing
on Xbox one and series X/S separately, I can't confirm that it will work a 100%.

Thanks again. I appreciate the help

He would need his own account. You can't sign in and play on multiple consoles with the same account.