For the sake of clarity and a simplified answer:
Typically MTB front and rear have the asymmetric rims opposite from each other.
Put the 'fat' side of the rim towards the the hub flange closest to the axle center (this pushes the holes towards the center of the flanges so to speak).
On a disc front wheel, the rotor mount takes up the most space, pushes the flange in and therefor takes the 'fat' side of the rim.
Opposite for the rear, where the driver typically takes more space that the rotor mount, so the 'fat' side goes to the cassette.
Below left is what you are after for rear, opposite for front.
Seeing you asked this question, I think you will benefit from this build guide. It goes through a few concepts, and then later the specifics that would apply to get it right for an asymmetric build.
This is the reason for my answer as the wheel will be flipped and flopped as you are finding your way with the spokes and then it's easier to recall "fat side - sort flange" logic IMO.