You don't need to be an 'investor' to invest in Singletrack: 6 days left: 95% of target - Find out more
I shall like this set up. It’s on a La Pierre explorer.
How does that single fixing rack work? I’ve seen them around and can’t figure why they don’t rotate.
I have looked on the La Pierre website with no luck.

They have a metal strap that runs underneath the mudguard to the seat stay bridge. One bolt runs through the strap and mudguards to the underside of the bridge. My Trek commuter bike is the same.
Looks like this is what's under the mudguard. Suggests it runs right down to the bottom bracket, I'd either forgotten that or not realised.
I've got a similar combo for a Cube hardtail I use as a dad/commuter type bike
@munrobiker has a particularly neat and low riding rack and mudguard iirc.
Cube acid
On my bike (la pierre) the rack mounts are on the inside of the frame with a shaped indent. Is this detail some sort of standard for a specific rack?