Why is whole bathroom backed up and toilet is coming up into shower as well.

Your main drain is clogged . Toilet and Shower drains connect into one, common drain, under floor. The blockage is past this connection. Therefore, when you flush toilet, water has no place to go but out the shower drain. You have to snake your sewer.

U have blockage in waste after the shower enters.

Snake through toilet and about 10' more distance to shower drain. Flush shower with hot water.

Are u on city sewer or septic. If septic, may need cleaning.

Good luck

Chuck

Let me finish what Milo's started. Locate the lavatory roof vent, rent a sewer snake,(see image) and send it down it. Put out enough cable to reach the base and 20 feet more. Good luck, Tom