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> To test these predictions, we conducted a double blind, sham-controlled, within-subjects experiment using EEG and high-definition tACS (HD-tACS; a form of tACS that provides more precise targeting of cortical structures)

So they're applying stimulation with electrodes placed on the head. tMS and stuff like this are the main ways available right now for non-invasively inducing oscillatory activity.

Not sure about this article, but there is some work using ultrasonic transducers that I don’t see mentioned frequently. (Magnetic stimulation has a longer history.)

I don't think we can call electrodes on (in?) a shaved head 'non-invasive'...

it is 'on'. And so, yes, we can:

https://en.wikipedia.org/wiki/Non-invasive_procedure

Well, since I don't consider getting a haircut or shaving to be a surgical procedure, I'd disagree.