Just like you might say that a lawnmower is compliance tested against electrical faults before sale and is thus safe to use, despite having some sharp rotating blades on the bottom that could ruin your day pretty quickly if you put your head in it and turned it on.
The way EM radiation works is that it's classified as ionising if it's sufficiently energetic and penetrative (read: high frequency/short wavelength) to enter human tissue and detach electrons in the atomic structures that make up that tissue.
The danger that ionising radiation presents is that detaching electrons in DNA can cause carcinogenic mutation i.e. cause cancer.
Radiation in high-frequency ultraviolet and above in the EM spectrum is considered ionising, and everything from low-freq UV below is considered non-ionising. This is because the energy carried by EM waves of lower-than-UV frequency is not sufficient to ionise atoms in human tissue.
I don't know the exact numbers on that, but I do know that the lowest threshold for ionising any atom is 3.9eV (low f UV) to ionise a caesium atom — and obviously we aren't made of caesium.
And this is backed up pretty well by the fact that if visible light were dangerously ionising EM radiation, life would not exist as we know it, and even if it made it this far, we'd all have a lot of cancer.
And since visible light is ostensibly higher frequency/higher energy than this "terahertz radiation" (presumably a buzzword for high freq microwave/infrared?) we can safely say that it won't hurt us in the way that a thousand chest X-rays might.
Now it's true that you can burn your food in the microwave and you can blind yourself with a laser, but this isn't because you ionised your chicken beyond core cooked temperature, it's because you subjected it to intense microwave radiation which excites the atoms.
Sufficiently intense visible light will burn your skin, but it won't mutate your skin. You get skin cancer from excessive exposure to ultraviolet, not from light.
Cellphones, wifi and 4G/5G have all been used by those who profit from scaremongering, but if you think about it, if any of it were true, there would have to be some new science to describe the mechanism by which they cause harm.