Why is there a "Y"?

I'm a first time poster here, avid DIYer and need a little advise on dealing with clay pipes! So the short of the really long story is the drain pipe from half (yes only half) the house is cloged with roots. We moved into the house two weeks ago and found this out during our morning showers. Of course the drain in question is outside of the foundation, clogged with roots and therefore not covered by any warranties. The plumber wants a lot of money to dig up the pipe and I said no I think we can do that so a digging we went. I found the pipe and started tracing it and the ground started getting very wet. Thought I somehow punctured a waterline but could not figure how I could do that with a shovel. Well it was not a waterline (thankfully) it was the upper portion of a clay pipe "y"? why is there a "y" in the pipe and why does it not have a cap or something but in reality those are curisoity questions that will not effect the fact the pipe will be replaced! So this drain also connects to the kitchen and we know it IS draining just really slowly but now that we have uncovered the open "y" well you know where the water is going!

So my true question is how do I plug the clay pipe temporally while we finish digging the rest of the pipe out for its replacement and still use the drain for the kitchen?

I should also mention we have a septic tank (9 ft from back of house) that was drained the month before we moved in, the house was empty for 2 years, and we have very rocky clay soil.

Thank you for any suggestions!

You have several questions but we do not have enough information to answer them. The Y could be ther remnant of a cleanout which was damaged at some time.

Sorry about that my only real question is how to I plug the clay pipe? What is the part called that I'm looking for? Is it just a "clay pipe plug"?

Thanks

If it is an unused branch, it can be plugged and sealed with mortar. Many plumbers would replace any sections of clay pipe with cast iron or PVC, as the clay is likely to have leaking joints which is where the roots come into the pipe.

They do make a "clay pipe plug" which is just a clay disc you drop into the "Y" and then apply mortar around the edge to seal it.