Most large US companies if they want to have the government as a client, have special rooms with employees with security clearances that work mostly on projects that the CEO and even their line managers do not know what they are. HR also have a special employees or a private section that deal with these people. There is some deniability also: the managers can tell the truth under oath and say they have no knowledge about the projects!

It's part of normal business operations not some secret illegal shady kick backs operation within some kind of spy movie.

There's no need to secretly infiltrate a company when the company is working happily with you. People underestimate how close big business and 3 letter agencies actually are in many cases.

The US intelligence agencies would have a hard time planting employees in US firms while remaining within the bounds of FISA. They certainly have assets inside major Chinese firms though, of course.

It doesn't quite work like that. You don't go off and find some super smart person who can easily get a job at Nvidia and tell them "Hey you, go work for this company". You find someone who already works at Nvidia, and you bump into them at their kids' little league game, and you get chatting and build a rapport and over time you build trust and before you know, you've got a trusted source inside Nvidia. They don't even need to know they're a plant. And then, if you really do need something more than information you can go further. But you start with someone already at the company and you target them, rather than the other way around.

And that's not even talking about the less clandestine methods of just ringing their General Counsel and saying "Hey buddy we're the US government we need X Y Z".

They do, according to Jonna Mendez, but those placed don’t collect a company salary, only a government salary. This causes some contention since they are paid far less than salaried employees and get no equity, so it leads to high turnover