Structures built over subfloor in loft

For the loft space, we are considering of installing Egger Protect 22mm chipboard on the floor. If not that, due to lack of availability, maybe 18mm plywood.

The floor consists of joists interspersed with steel beams.

We will then build dormer cheeks and fronts and also ashlar walls supporting the rafters, as well as standard room partition walls. Some of these supports coincide with the steel beams (that is why the steel beams are there).

If we lay down the subfloor, we will have covered over the steel beams.

What is the proper way to build in the loft, floor first or structures? Or a mix and match?

I attach a diagram, red lines are steel beams in the floor.

Just to answer my own question after talking to engineer, all structures need to go over joists/noggins and not over subfloor. It is more work that way but you can remove the floorboards in the future without cutting or breaking.