Do I install the floor first and then the walls?

In any case, you always start by placing the walls first. Afterwards, you can install and finish the floor structure for each room.

The reason for this is to acoustically separate the rooms from each other. For example, if you first install the floor and then build the walls on top, the sound from one room can very easily transfer to the adjacent space.

A second reason for this is that it is impossible for us to estimate what the walls will be made of (wood, metal studs,...) and how much this would weigh on the floor.

That is why we always recommend installing the walls first, and then installing the floors in the created spaces. This is also standard practice with screed floor constructions.

Optionally place edge insulation between the floor structure and the wall to achieve even better acoustic performance.