A two story waste and vent job can look like this.
Vents are tied in 6" above the flood level of the fixtures served.
You will note that the lower floor vents tie in at 6" above the next floors plumbing fixtures's flood level.
No waste lines are used as vents.
It would defeat the purpose of the vents.
You really only have one chance to have it right.
I hear so many stories of bad plumbing behind walls, making it difficult to repair and make right.
Notice that the vent for the 1st floor tub ties in on the second floor at 42".
The waste line for the toilet is the only one that does not have a p-trap. The trap is internal to the toilet.
If you look at the side of a toilet, you will see the curve that goes first up, and then bends down is the trap. The high point of the bend, becomes the level of water in the bowl.
If the vent can't go up, because you are on an island, then some codes allow the vent to go back below the floor. This is done by tying the vent back into the waste line using drainage fittings.
However, before it ties back in, a "foot vent" must be added.
A foot vent will run from the loop until it finds a vertical area so it too can reach for the sky. Where the foot vent turns vertical, you will have an accessible clean out.
The foot vent won't do much good if it becomes blocked. A clean out will help if that ever happens.
All fittings below are to be waste fittings, things like wye's on the horizontal and proper bends.