Bad smells like a drain

Hello, about 2 years ago our house was handed over and shortly afterwards the bathroom started to smell unpleasant from time to time.
At first I didn’t think much of it, since it smelled damp in the rooms occasionally during the drying phase anyway.
But now this smell has honestly been driving me crazy for months.
It can be detected at the washbasin and recently also in the shower and the bathtub directly at the drain.
A slightly rotten, musty or moldy smell fills the entire bathroom after a while and spreads through the whole house (bungalow) when the door is open.
The plumbers have already been here and couldn’t find anything.
The trap at the sink works, also in the shower – nothing is being sucked dry. The drain piping at the washbasin has also already been replaced.
Over the weekend I thoroughly cleaned the flat shower tray with cleaning agents ….. but again there was a musty smell yesterday.
I really don’t know what else I can do. I have also browsed various forums, etc.
An extensive cleaning, with or without chemicals, has unfortunately not brought any success so far.
Do you have any ideas?
Could it possibly be related to the roof ventilation?
Can sewer gases rise back up through the trap water due to over- or underpressure?
Regards

Something similar happens with us when we don’t use the bathtub for several months, then the siphon dries out and the air path is obviously open. But I can quite clearly smell where the odor is coming from.

then it is clear why it stinks. with us, however, everything is in use daily!

We had a similar problem. With us, the shower drain had to be pressed down extremely firmly, only then was it airtight (it was already watertight before). Sometimes it can also start to stink at the overflow drains of the bathtub (these usually do not have a siphon). In that case, chemicals should help,...

Well, the fitting in the shower is tight, even tighter and there would be a hole in the cup - no, actually I'm not in the mood to joke ...... that stinks terribly .... but even my plumber (buddy - didn’t install it) doesn’t have any advice. In the bathtub, there's probably still water in the drain further down, right?

Is there still a floor drain in the bathroom?