There's two ways: Modify a Miata; Make something look like a Miata.

Most would think it's easier to modify a Miata. That's sorta true. One could relatively easily replace the suspension, rims, and tires to make it look like the OP. But, the first time it's off-road it'll likely bend the frame.

The frame needs to be reinforced to function off road. That's generally a huge pain in the ass. It's much easier to shorten the frame of an existing SUV or light truck then attach the Miata's body and panels to it. It's much easier to make a truck look like a Miata than make a Miata into a truck.

But, this particular vehicle is very lightweight. Frame reinforcement might be much easier. A Miata hobbyist or Mazda mechanic would know because there's likely only one way that makes good sense for this specific model. My best guess is that because the Miata frame needed very little it was chosen for this purpose. I'd guess it's a modified Miata.

Next time ask the owner anything. Everyone who owns something like this is dying to tell others all about it.