What is the technology behind Hank Pym controlling ants?
In this scene we see Hope controlling the ants,
What is the in-universe tech for this?
The quote from the movie is
Hank: I use electromagnetic waves to stimulate their olfactory nerve center.
Since ants communicate via pheromones, Hank Pym uses special EM pulses to fool them into thinking that they're smelling what he wants them to smell, down to giving them very specific commands.
If you want more info about the device (and how it was invented), you can find it in Ant Man: Larger Than Life, a one-shot comic produced to prequel the Ant Man movie.