I would imagine that this would first manifest itself in smaller countries. For example Mali employs the Wagner group out of Russia as a supplement for its Army. And I believe they are used by other countries as well.

I don't see nuclear-armed nations using private companies, but possibly being the source of troops and technology. Really I don't see U.S. citizens joining mercenary groups, quality of life is too good. I would imagine 2nd world nations like Russia, Iraq being the main source of these troops, backed by cheap advanced armaments from China.