Do I put the floor first and then the walls?

In any case, you first start with placing the walls. Afterwards you can place and finish the floor structure per room.

The reason for this is to be able to acoustically separate the rooms from each other. For example, if you place the floor first and the walls above it, the sound from one room can easily transfer to the next room.

A second reason for this is that we cannot possibly estimate from what the walls will be made (wood, metal studs,...) and how much this would weigh on the floor.

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

If necessary, place edge insulation between the floor structure and the wall to score even better on the acoustic level.