I think I know what you’re talking about- Typically plumbers will just stub up thru the concrete with a piece of pipe -after the rest of the concrete is poured the plumber will come back & cut the pipe down to the proper height& install a toilet flange- many will leave this part out all the way to the “finish” part of the plumbing process (you know, when its time to set the sinks & toilets. You could do this yourself but I recommend you hire a plumber to do it right & while it won’t be exactly cheap, It won’t be terribly expensive either.