What is the model of the heart rate monitor?

Is it able to measure continuous HRV?

Yes and no - you can get all the absolute heart rate values for each heartbeat so can work out HRV pretty easily with an app. HRV calculations are not built in to the firmware though.

Does it measure SpO2 (blood oxygen saturation)?

The VC31B is capable, but it's something provided by a binary blob which we don't include in the firmware. So no, it's not possible yet.

What is the quickest frequency at which it records heart rate?

You'll get an event for every heartbeat. Right now the PPG is polled at 25hz (or 50Hz on the VC31) which seems fine, and it is possible to raise that to ~100Hz or more but on the VC31B that functionality isn't part of the firmware and requires some register poking.

Is it a PPG or something else?

Yes, it's PPG.