Installation of heat pump/heating - before or after screed

Yes, it depends on the composition/type. Each screed has its own temperature profiles.

By the way, it is cement screed without additives. I suspect it all depends on the type of screed and the additives used when and how heating should/may be done?

Yep.
There is a heating-up protocol available for the screed.
Example for standard cement screed (although with today's flow temperatures, the 55 degrees is more than questionable - it seems to settle that 5 degrees above the maximum flow temperature is sufficient):


With additives, e.g.

There is so much talk about dry heating here. I thought functional heating is meant to test the functionality of the underfloor heating and not to dry the screed. On the contrary, actually drying the screed too quickly (removing water) is not desired, because the water should bind through chemical processes and not just evaporate. That is why there are these "careful" heating protocols. Otherwise, one could just turn up the heat to see if the heating works and that would be it.

A lot is said here about dry heating. I thought functional heating serves to test the functionality of the underfloor heating and not to dry the screed.

Of course, both apply and nowadays both are done simultaneously to save construction time and money as much as possible.

Functional heating takes place according to our screed layer to relieve stresses from the screed and thus avoid (hair) cracks. For example, I was also surprised that we initially do not have to heat at all (which would have been quite difficult without heating). But what did people do before underfloor heating? Let it dry for a winter. So if you have time, you might even save money since this is currently probably done with electricity (heating rod) and thus causes considerable costs. Independently of each other, energy consultants, plumbing, and the drilling company warned us against heating the screed with the heat pump in order not to extract too much energy from the borehole in a short time.