Ok, I got confused here, so excuse my ignorance here, please. From what I can tell, the disc included in this new 'Xbox' case will be able to run on the Series X, Lockhart and One without any issues of compatibility? Won't there be separate discs for each console from now on?
Pretty much. You can currently insert a ps3 disc into a ps4 and it can detect that it's a ps3 disc and not play it. The concept is essentially the same. You insert your game into the Xbox One and your Xbox One will run it. You insert the disc into the Series X and the Series X will run it. The game data will either be in the disc itself or it you will need to download additional data like 4k texture packs and etc for the Series X.
I could be wrong but the One S, One X, (maybe) Series S and Series X all should have UHD bluray drives so... Shouldn't have any issues.
PC Specs: CPU: 5950X || GPU: Strix 4090 || RAM: 32GB DDR4 3600 || Main SSD: WD 2TB SN850