It’s been a while since I looked at how Valve does it but it could be called a primitive expert system. And while the HL1 grunts were extraordinary for their time, HL2’s combine grunts are still pretty much the gold standard. Without the AI leaking information to the player via radio chatter it would feel very much like the AI is cheating because yes, HL2’s grunts are better at tactics than 99.99% of humans. It also helps that you’re a bullet sponge so them outsmarting you, like leading you into an ambush, doesn’t necessarily mean that you’re done for.

OTOH they’re a couple of pages of state-machines that would have no idea what to do in the real world.