Global set music on hold

I’m trying to wrap my head around how hold music is selected. I see there’s a setting of the ‘default’ category for all inbound routes, but I’m not clear about when that setting is chosen: Is it just when that route has been used to initiate the call, or is it any time the extension with a DID-based direct inbound route puts a call on hold?

Also, is there a way to change the category for all routes at once?