2.83: how to join cylinders and hollow out the junction I want to create sewer pipes where a player can walk inside on a 3D game. Then I'm trying to make them by multiple cylinders like this.

Now I have two problems. First, I don't know how to cut A's edge.

The other is how to cut B's side  faces.

I tried to do that by boolean modifier, but it didn't work. So will anyone tell me how to do that? Thanks.
This particular issue arises in the old boolean solver.  The issue is that these shapes are exactly the same size and their points are coplanar.  The solver doesn't correctly eliminate all the faces.

2.91 introduced an exact solver that can be activated with a button in the modifier box, it is for these situations.