How are flat roofs sealed?

Hello everyone,
how is the sealing of a parapet actually done if a railing is to be mounted on it? I started drawing as follows:
[ATTACH alt="attika.jpg" type="full"]39470[/ATTACH]

On the Ytong stone, there should presumably be a concrete frame (which should possibly be 10 or 15cm high) in order to mount the railing on it. How is the sealing done here approximately? Do you then drill into the sheet metal later (unless you possibly use a granite slab with a slight inward slope instead of sheet metal)?

Info: I spoke with 2 waterproofers today. One drives the vapor barrier only sideways onto the Ytong block. A concrete belt is still planned on the Ytong block. The second waterproofer would place the vapor barrier on top of the concrete belt.

Regarding railings, there are also different opinions. The first waterproofer says that a railing on the parapet is no problem. There are special (I believe plastic) seals for the railing screw connections. The second waterproofer strongly advises against that (a side railing would be the safest).

How do you see it regarding railings and seals directly on the parapet?