Hey Nisarg...
1. Both! It should be a joint effort between both with equal participation from other stakeholders as well.
2. It is ideal to do a D90 survey for new joinees which have a different set of questions than your regular eNPS. Since the focus areas are different for a new joinee than a tenured one.
Once these focus areas are defined, frame questions which make sense at various stage on the journey.
Run surveys at D1, D7, D15, D30, D60, D90.
Analyzing these results at a team level can give great insights on which Focus area is going wrong (or right) and take corrective measure.
Here is an example ->