How are prototype AFVs constructed?

[raketenjagdpanzer]

There's a restoration effort planned for the Chrysler/GDLS XM1 prototype, and I have seen it (and the XMBT-70) referred to as "mild steel", so that's...what, like the stuff a dumpster or car is made out of?

The reason I ask is, IMC I may have a few one-off prototype show up in the hands of CivGov troops either in Europe or back in the 'States...probably Europe since I think per canon there was one last reinforcement push in '98 by CivGov and I can imagine them cobbling together everything they could, including raiding FMC's facilities and Ft. Knox for the XM800 ARSVs, getting the engines running and guns in working condition and sending them on, but not if they were just mock-ups.

(ps yes I know but it's a game...)