Dual boot on new computer

If this is not the place for this question, please let me know without flaming me. And point me to the correct place. It’s been a while since I’ve worked on these issues so bare with me.

Just bought my son a new pc and it runs Windows 11. He has concerns about running some of his games from Windows 10. The main one that Windows 11 will delete his files.

Can I plug the old HD (with Windows 10 already loaded) in along with the new HD (with Windows 11 already loaded) and have it dual boot without crashing both HDs? Is there a tutorial that explains how to do this? Would this prevent the boot problems I’ve read that Windows 11 has?

Any help would be greatly appreciated!