This probably sounds real dumb to you, but I'm planning on rebuilding my basement bathroom in the near future and having problems wrapping my head around the following:

Before building the shower or placing the bathtub, I'm going to be burying underground 2" pvc. The trap obviously will be below the floor and probably will be glued together schedule 40 right? Instead of the thin plastic p-trap crap? Because it's underground.

Anyway at the end of this I'll have a vertical pipe sticking up thru the concrete. Let's say it's in a corner or an alcove. When I place the bathtub in position, I'll no longer be able to access the area where the tub drain assembly connects to the 2" pvc.

So how in the world am I going to make the final connection? How do I tighten down the slip joint thing, if indeed that's the way to do it?

Can someone briefly explain that or maybe give me a hint or where to research this.

Thanks!

If you are using a shower, you use a drain that lets the pipe stick up thru. After the shower is set in place you cut the pipe off and install the friction ring and nut to form a watertight seal.
If you are using a tub, you just install the waste + overflow thru the stud wall before hanging any sheetrock.
It's better to build a little square box and put it around the pipe and fill it with sand before you pour the floor, then after the floor is poured you can remove the box and cut the pipe below the floor level if need be, and you can offset the riser out of the trap if you miss your measurements by a couple inches.