What are some causes of sewer gas odor and how do you fix it?

The master shower smelled and the vent on the roof from that bath smelled. All the sinks/toilets and baths she uses so I don’t think it is caused by evaporation. If a trap is self siphoning to let the odor in the shower how do you fix that? What other items can cause sewer gas smell?

Does the house have a crawl space? If so, I would look for and S-trap configuration (or no trap) below the shower floor. If the house is a slab-on-grade or the trap area below the shower is concealed between floors you may to be able to tell, but it could be that the shower has no trap

Yes, hidden between floors…

The vent should smell, as this is a direct, open connection to the sewer/waste system, hence the requirement for vents to terminate outside and above the roof line.