The P trap is the U shaped part of the piping under the sink or below the floor in the bathtub. (I know, it should be called a U-trap.)
Look under you sink and you will see it. Every sink and tub in the house has one. The toilets have big ones.
The "U" shape is always full of water and prevents smells from the pipes and sewer from coming up into your house.
I think any smell you are getting is coming from the P trap or the short pipe connecting it to the tub or sink. The water in the P-trap would stop any smell from coming from further below that.
The P trap often has gunk and hair in it that could hold smells. And as it is always full of water, it is hard to rinse out. It's possible to flush it all out if you put a lot of water down, but if there is gunk it there it could be holding smells.
If your plumbing isn't too old, cleaning the traps is an ugly and dirty job, but not a hard one.
Assuming easy access, a skilled person could help you do them all in an hour. He takes them out, someone clean them *perfectly till they don't smell*, he reinstalls.
