The UFH is normally clipped down and the screed is laid in on top of that. On larger jobs the UFH is sometimes laid out to avoid the lines of stud walls, hand rails, etc with tracks between rooms going through doorways or pre-defined paths beneath stud walls. On smaller jobs it is just laid down over the whole. Either way you need to know that a 50mm screed really is 50mm thick and to warn any following trades what depth to expect. It isn't much fun when the screeder has put insufficient depth of screed in and your guys have been told the UFH is at 70mm only to find it is at 40mm... Very expensive to sort out, although the "fountains" in the floor that you see when they pressurise the system look "interesting" to say the least (and before you get the wrong idea this has happened to me only twice in the last 20 years, each time the screed was sub-spec)
I've put up stud walls (metal track) then boarded them before a dry screed (70mm, commercial) has been put in quite a few times. The mix used we can normally walk on within 24 to 48 hours. Works well enough for us. With liquid screed it seems to be normal to run the screed in before the stud walls in place as the bottom of the plasterboard would just turn to mush if they were already in. The main thing is that you need to be certain that any soleplate fixings (for walls, etc) do not pierce the UFH - so if the spec calls for 40mm, check every time you drill to ensure that you have left 10 to 15mm of leeway and make sure that any trades coming in to that room know what is under the floor (BTW this is another good reason to use MF over timber stud - MF can be power nailed in with 25mm concrete nails which you know won't go anywhere near your UFH whereas every hole you drill is done by a human being, allegedly)
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