Plumbing – Sewer pipe cleanout

Here's my speculation, based on your description and my experience. It is just a theory. Do not commit any resources based on this without collecting corroborating physical evidence supporting it. A direction to investigate at least.

The main floor plumbing goes out to a septic tank. The pipes were not placed lower because there was no need, it just meant digging deeper outside. No one wants to do that without good reason.

When the city sewer was installed, the basement construction tied into it, but the old plumbing was never redirected, it remained on the septic tank. Not the best scheme and possibly illegal, but I'm not surprised someone did this.

It is highly probable there is nothing below to the basement to tie into. The sewer may not be much lower than the basement drainage exit. Anything placed lower than this will likely need to drain into an ejector pumping chamber.

Your system is almost certainly vented. Plumbing vents provide a path for sewer gases to escape so they don't bubble up through the P traps into your house under pressure (the sewer gases will expand and exert greater than atmospheric pressure), and the vents also keep water pressure from siphoning your traps dry, which would create an easy natural path for sewer gases to infiltrate your home.

So the comment about "wet venting" is right on target. As long as the combination of pipe diameter, the slope of the pipe and the distance between the trap and the vent ensure that the pipe doesn't run full of water (creating a siphon), a single downstream vent can be just fine.