Suggestion needed for combining the custom objects

Hello Everyone,

Another Beginner level question from my side.

I am creating a game consisting of cylinders, pipes, objects that fits into each other. I will need suggestion and help how to achieve the following things:

1. In order to combine two objects, how do I keep the combined objects ? I am allowing user to touch the individual objects and create one major thing by combining cylinders, pipes etc. So, when user drops a cylinder and then again drops a pipe, and try to move the pipe towards the cylinder, how do I keep the combined object on screen/game? Eg. Pipe stuck into the cylinder.

2. Another question that I have is, I want to know whether it is possible or not, to allow user to move the pipe inside of cylinder from one side only (i.e. where the face of the cylinder is open)?

Thanks Everyone, Lokesh Sehgal