Will Photoshop 6 run on Big

No, not directly. You'd have to first install a VM, such as Parallels, VMware or VirtualBox. Once that's on the drive, you install an older version of macOS in the VM CS6 will run under. Nothing newer than High Sierra would be best. Lastly, once the OS installed as a guest OS in the VM, install CS6.

Since you're running an OS within an OS, you should have 16 GB of RAM in your Mac as the bare minimum. 32 GB would be better. Also, a VM literally means a virtual machine. It emulates a computer within your computer. The biggest loss is that none of them emulate high powered video hardware. Expect graphics intensive operations to move slower within the guest OS.

ILCwarksUK wrote:

I did think Big Sur had the way of running old programs through something called Rosetta (?), or did I dream that?

Rosetta 2 allows Intel apps to run on Apple Silicon-based Macs. Not 32bit apps on 64bit only macOS versions.

Consider evaluating the full trials of the Affinity product line. All items are half-price ($24.99) in the Mac App Store, and are non-subscription. Don't let the price mask the feature set in these applications.