A piezo tweeter can be basically modeled with a capacitor in series with a resistor. The piezo element is essentially capacitive. The series resistor models the losses.
The impedance of any speaker can only be given as a graph vs. frequency (and even multiple curves, vs. power, but let's keep it simple).
Piezo tweeters typically have a pretty high impedance below their cut-off frequency. The range for which the impedance is more or less constant (it isn't) depends on the specific tweeter, but it's usually something between a few kHz to a few tens of kHz.
They absolutely do not have a voice coil, that's one of their plusses for high frequency response. Basically, it's a piezo element that moves a cone of some sort.
Something to read:
https://pispeakers.com/PIE_Speaker_Application_Note.pdf