MEMS piezoelectric sensor circuitry
It can contain whatever the designer wants it to.  As pointed out in the comments, the usual commercial chip has on-board signal conditioning electronics, to the point of having a digital interface.

I have worked on signal conditioning for a silicon MEMS gyroscope that just brought the capacitor plates out to the pins, with all of drive and signal conditioning happening off-chip, but that was specifically being done in an early stage of product development.