@Speedranger53 Basically, the Series X version is a different game. It will be 128 players on larger maps. Xbox One will be 64 players on smaller versions of the maps.
The only way I see Xbox One/PS4 being able to play with Series X/PS5 is in the Portal. The Portal lets you set parameters for whatever you want in the game. That means a Series X/PS5 player could create a 64 player lobby or less. But that could get complicated because the game would have to evaluate each lobby to see if its compatible or not.