You just drag them from the existing disk to the new disk in File Explorer, but maybe there's something more complicated about what you are trying to do? At the simplest level, you have both disks connected to your system, and just drag and drop. If that's not a convenient solution, or there's something more specific you are trying to do, you need to give a little more detail to explain what you are trying to do.
If you can't have both disks attached at the same time, copy from the existing disk to an external drive, then from the external drive to the new disk.
So, this is a second drive, in addition to your Windows (C:) drive?
In the Xbox and Steam apps, you can sometimes move games installed via those apps to a different drive. It looks like not all games installed via Xbox can be simply moved, but I think all Steam games can be moved. Just right-click the game, then there's Manage/Properties options and files tabs where you can move things.
It looks like the Epic Games app does not have a feature to move games to a different drive.
For standalone installs (i.e. not installed via one of those store apps) and Epic, reinstallation and choosing to install on your new drive is probably the safest and simplest way to do it.
For simple documents, photos, screenshots, miscellaneous data, etc; just drag and drop in File Explorer.