Why is it that sometimes when showering the shower backs up and the toilet gurgles and the bathtub has some backflow and in the other bathroom a sewer smell is detectable?

Assume you have had the main drain rooted out so the waste has clear passage out? Sounds like your vents are clogged. Spiders make webs, leaves get in the web. Rats go down vents and get stuck. Assume Plumbing worked correctly once upon a time. If not maybe built wrong. Drains can share vents but they can't tie together below the flood line of a fixture. Except toilets on the same floor. Wet vent is exception- all fixtures same floor, oversized pipe by factor of one main no less then 4 inch, all fixtures joining in are sand tee style fittings. That maybe Greek to you but maybe the Roto Rooter people can look for those problems while under your house.

AnswerAlso the smell comes from your pee traps draining-clogged vent. Or illegal (in CA) S trap set up.