On the Untamed documentaries, most of the workers were American. These are actual RMC crew. Granted, I'm sure they brought in some European help.
RMC does most of their own work, hence why they have their own trucks and employees. If I remember from an interview with Fred Grubb, the number of employees they have varies based on how much work needs to be done.
The one employee is female, and very recognizable. I remember seeing her on Untamed construction, and I think Zadra construction.
GCII has their own crew too, of official carpenters. They were the ones building Wickerman and other overseas rides, no doubt.
Intamin, for example, has no construction crew, hence why companies such as RCS, who have experience, are often hired. I mean, pretty much every steel coaster built in Europe is by RCS
RMC got their work in roller coasters started by constructing T Express and El Toro, as well as other, mostly wooden rides before breaking into their own.