My two upstairs bathrooms will bubbles and then backs up every time I run the faucet or shower in the upstairs bathrooms. I've tried plundering it and tried the snake as well... nothing seems to help. In dire need of advice!

You haven't " plundered" Halloween joke? Plungered enough or snaked enough. You have a blockage in the line after the sink drains. Auger through the wall connection at the P trap.

Stupid question but what's a P trap? I also read online about the vent being blocked? Is that possible?

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Got it! Thanks! I will try to auger that and see what happens.

You have a blockage down stream from the connection of the vanity or shower drains that can't be reached from the trap. You must either snake from the lavatory roof vw3nr or pull the toilet and snake from there, Good luck, Tom