Ultherapy uses ultrasound targeting the 4.5mm SMAS layer to stimulate collagen...

https://technivorz.com/why-is-imaging-such-a-big-deal-in-ultrasound-skin-tightening/

Ultherapy uses ultrasound targeting the 4.5mm SMAS layer to stimulate collagen deep under the skin, with results peaking around 3 to 6 months. RF heats tissues to 40-45°C for surface tightening but doesn’t reach as deep