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So I get the concept to be able to work on modeling and animation at the same time by switching the pose position but what are the copy constraints linked to?
It’s hard to tell from a video, without digging around in the actual rig, but it seems like he used constraints for connecting/parenting his armature rig (which might’ve included the bones, foot controllers, and the local controller) and constrained it’s local controller to a separate object/s (or armature) used for global controller/s. Something along those lines. Possibly, used constraints for parenting all the controllers to each other in a hierarchy or just the local to global? Dunno.
You can use constraints instead of the simple parenting hierarchy, for parenting your controllers underneath each other. Seems like he’s using constraints for parenting to utilize their visibility toggle feature, bringing the child objects back to their non-rotated-funny position.
I don’t like binding controllers to each other with Child Of or Copy constraints, unless I have to. But I guess they can be useful at times in production.