Playing Xbox X on Xbox one

Xbox One and Xbox One S are 1080P Gaming

Xbox One S will play 4K Blu Ray's

Xbox One X will play Game in 4K if they are Enhanced with 4K Support + 4k Blu Ray support

They Xbox One X will also Upscale on a 1080p screen so you will see some benefit of the Xbox One X

Yes,

Anyone one can be on any of the...

Xbox One,

Xbox One S,

Xbox One X,

Xbox Series S,

Xbox Series X,

and join party's and play the same game together, no matter who is playing on any of these consoles.

Looking at the Xbox One X Enhanced list here:
https://www.xbox.com/en-us/games/xbox-one/xbox-one-x-enhanced-list, Doom is 4K on the Xbox One X, but not HDR supported.

If your Xbox one X is set for 4K resolution, then it will automatically play Doom in 4K

Xbox series X will be the next console of the Xbox franchise. Like the jump from Xbox 360 to Xbox one. You will be able to play you're Xbox one games on the Series X and the games coming out will be out on Xbox One as Xbox series X for a limited time.
After a while they will quit making games for Xbox one and focus on Xbox Siries X

The Xbox One can't play games at 4K, You need to have the Xbox One X and a 4K TV or a 4K Monitor to get to play games at 4K and games that show Xbox One X Enhanced on them.