I bought a house with an unfinished basement and three drains capped off in one room. I was told this wasd to add a bathroom. Next to the shower drain was a cinder block buried in the concrete filled with gravel. I was told this was to install the P trap for the shower drain. I busted out the cinder block in the concrete and dug around and looked with a flashlight under the foundation. The drain pipe from the shower is going into a WYE drain and the other end is capped which was under the block. Do I cut the drain under the concrete and install the P-trap immediately under the drain and tie into the line? Why isn't the p-trap just installed while the builbers are building the house?

You're using the word "drain" in a way that confuses me. In orger to help we have to get together on terms. A drain to me means the part the water drains into, like a floor drain or a bathtub drain. What's a "wye drain"? Do you mean a drain line,(niot to be confused with "drain") has a wye cut into it with the branch capped off and that's what you add a trap and raiser to? When we rough a unit we always rough in a "P" trap for future use.
The cement block was left there as a bathtub dap-out,( It does set about 15 1/2" off the wall doesn't it? However you can cut out the cap and prime and glue on a trap and bring the raiser up out of the floor a few inches. You now have the option of a tub or a shower. If you decide on a tub then leave the dap-out but if you're going to install a shower bring the raiser up and fill and cement over the dap-out. Will this be a custom tile shower or a manufactured base and enclosure?

From the pipe through the concrete the 2" spout gous into a Y shaped line under the concrete which is horizontal. Heading back towards the hole I broke through is a 45 degree fitting then a short piece of straight pipe capped. If I understand you correctly, there is already a P trap in this drain and the block I busted out was for an overflow drain for a bathtub? I am making a custom shower, so I can just fill in this area with gravel and concrete over, is that correct? How can I tell if these drains are tied to a vent?

In addition to my last comment, does that mean that the 2" pipe coming through the floor is supposed to be hidden in a wall at the end of the tub and tied into a vent line? The 2" pipe coming through the floor is 26" from the side wall and about 60" from the other side wall. Since, it is 26" from the side wall it is probably too far to be a bathtub drain. The other drains I know are for the toilet and sink.

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If I understand you correctly, there is already a P trap in this drain and the block I busted out was for an overflow drain for a bathtub?

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No, my apologies for making a misleading statement. I meant that when my company installs a unfinished rough in, whether it for a tub or a shower we always install a trap and raiser. Your plumber didn't.
Cut the cap off and install a trap and raiser on the pipe. Bring the raiser a few inches above the floor line and fill and cement over the dap-out leaving enough room to recess the shower drain. If this will be a custom tile shower you are now free to install a flange type shower drain, (see image) and the shower pan. Good luck, Tom

I think I have it figured out. Plumber did not install a P-trap on pipe next to dap out so I can't use it as a drain as I cut it and could smell sewer gas after pouring water down the pipe. I will have to cut the pipe off and seal it somehow flush with the concrete floor. I was thinking of stuffing toilet paper in then that urethane "great stuff and concrete over that. However, I can put a p-trap in the dap out and use as a drain. Also, since the sink drain is now in the stud wall I can T off that and tie in to use as a vent up above. Please, let me know if you see anything wrong with my thinking. As I am a relative novice.