A smelly drain is an unhealthy drain, all sorts of rubbish goes down your drain to the sewer system.
The system is designed to keep the smell from these biodegradable and compostable materials reaching inside the house.
If it does it means there’s a clog or blockage somewhere down the pipes. If the smell comes from the shower the usual cause is build up of hair in the drain trap, with kitchen sink buildup the usual culprit is allowing too much grease and food to go down the drain. Bacteria may get attracted to this build up and cause the bad smell.
Another thing may be that your water seal on one of your floor wastes could be dry, try tipping a bucket or two of water down every floor waste.