Well, the toilet's trap is in the toilet. Other than that, you really don't want to put that many angles in the floor under the concrete...better to jack up the floor and put things where they need to be by the shortest, straightest path to help prevent blockages. I'm sure there is also a limit on how many turns you can do under there, but I don't know the code. If the builder did things right, there should be vents for all of the rough-ins, otherwise, you'd have to rebreak up the concrete to put them in...you need to look for them. Each fixture should be vented, but they can be combined so you don't need multiple runs through the roof.