You will be able to play the your BC-supported games with your physical discs on the Series X, which has a disc drive. You pop it in and the console will download the game to your disk. With the Series S, all your digital purchases will carry over. So, for example, if you bought CoD: World at War digitally on your XB360, you can also download the game on the Series S/X. Those are not ports - the games are running on a virtual Xbox 360 GPU running on top of the hypervisor. It's really a marvel of technology and really encourage you to check it out.