Seal old building floor slabs and seal hole?

Hello everyone,

I am new here...
Lutz (carpenter) from MG and have bought an old building there.

The house has been extended over the past few years.
In the living room, three floor slabs meet exactly at the T-joint where the leveling compound (previous owner) was slightly damp.
I opened this spot and found three different floor slabs.
1. approx. 80mm concrete plus cast asphalt approx. 20-25mm.
2. approx. 100mm concrete
3. slag, lightly compacted/bonded sand, 30mm cast asphalt.

How do I best seal the hole?
Straighten the hole > insert insulation strip on the side > fill with ready-mixed concrete > seal the joint of the insulation strip with PU/silicone sealant > level with leveling compound?

How can I later create a base on the various substrates for our (already delivered) fully glued plank floor (oak)?

Level with leveling compound > seal and sand with epoxy resin > glue the floor?

Thank you very much
Lutz

A second picture

How do I best seal the hole?

Are any of the floor slabs sealed at the bottom at all?
I would additionally apply Remmers MB2K / WeberTec D27 including joint tape at the transitions over the entire area after your mentioned measures. - But that might also be overkill.

Oh, and: floor slab - living room: insulation?