Plumbing – Confirming if the toilet drain is properly installed

There is a clog downstream from both of these drains. If they are both at the lowest level of the house and you have a septic system, then it's conceivable that the problem is there, but you'd also see water backing up into that bathroom when you run any other showers/toilets in the house.

Most likely, the clog is in the joint in the drain lines where the shower and toilet join on their way out of the bathroom. Get a snake to fish down the drain (probably best to do this in the shower to avoid damaging the toilet bowl) and expect to go down quite a few feet before you reach the problem.

Edit: Revisiting this question, I'd agree that there's also a likely vent blockage from the bubbling descriptions. But the fact that toilet water backs up into the shower points to a drain blockage, too, and that will be easier to fix.

If you had "the sewage line get backed up" all along and didn't do anything to fix it (replacing the water line doesn't) then the sewage line will only get worse.

As for your tub (I think you want me to guess you shower in the tub instead of having both a tub and a separate shower), it probably connects to the sewage line after the clog that affects your toilet.