What exactly happens when ultrasound enters the ear canal?

Not much, unless the ultrasound is very intense.

At high intensity it causes heating of tissues. “The rate of heat production in an ultrasound field of intensity, I , is equal to 2αI , where α is the absorption coefficient.” At sufficiently high intensity, it would cook the inside of your ear in similar fashion to directing a microwave oven into your ear. Unlike microwaves, ultrasound would affect the bones in the ear first, as they are more highly absorbing of ultrasound.

"In a homogenous perfused tissue where the contribution of perfusion is relatively small (e.g. , bone), instantaneous temperature rise (ΔT) may be estimated using the equation Δ T = W/4 d6, where W is the total acoustic power (mW) and d6is the beam diameter (mm). The magnitude of temperature increase is time dependent, and is more pronounced when the ultrasound beam is directed at tissue with high absorption (bone and cranium). "

It’s also possible that high intensity ultrasound could cause fractures to occur in the bones of the inner ear.

Other possible effects of high intensity ultrasound include Thrombus formation (blood clots), free radical production, and changes in antioxidant enzyme activity. It is also thought that high intensity ultrasound may cause DNA degradation, though this is the subject of ongoing research.

Of course, at “normal” levels, none of these effects occur to any detectable degree.

There’s lots more information available from medical websites, such as https://pubs.asahq.org/anesthesiology/article/115/5/1109/12838/Potential-Adverse-Ultrasound-related-Biological

Not in the conventional sense.
The tympanic membrane vibrates in similar fashion to the diaphragm in a microphone. The diaphragm in a normal microphone will vibrate up to around 20 to 40 kHz, with much reduced amplitude at the extreme high end. Close to the maximum frequency, a microphone diaphragm will start to “wobble” and flex rather than vibrating in the normal way, but beyond a certain point it simply can’t move fast enough, so it does not move and no signal is produced.