Empty conduit telecommunication?

With us, the concrete worker laid the empty conduits under the floor slab, the extension a few meters away from the house, and then later through the slab into the utility room was done by the electrician, the rest by Telekom/providers.

It is too late for a conduit through the base slab, I assumed that one would have a multi-duct entry for such things.

I found flexible cable protection pipes for underground installation on the internet, or what is used as a "Leerrohr"? What diameter?

Normal PE pipe 80/100 DN50 or d63 is completely sufficient, weld end caps on both sides and that's all. If Telekom doesn't want it, ask the supplier for such a pipe, it is then blue or orange.