I'm not an expert, but I'll attempt to explain it as I understand:
I know what they are and why they're used etc. I just can't find any solid info about their design.
Air that is sped up has less pressure, so the tunnel design speeds up the air, sucking the car to the ground..
That's correct. Any time the air speeds up, there is an opportunity to create lift, or downforce, depending on the geometry.
Okay, is the tunnel 4 sided or is the ground the 4th side and side skirting helps maintain the seal?
The ground is the 4th side. This creates a tunnel that has a larger entrance and exit than the middle. Because the cross section of the tunnel is smaller than the entrance or exit, it speeds up the air velocity, lowering the static pressure. Since the top of the car is covered in normal velocity air, at a higher static pressure, there is a difference in the force on the top vs. the bottom of the car, and hence downforce (or lift) is generated on the car body.
I read about the inverted wing design implemented in a venturi tunnel. Then sometimes another tunnel seems to narrow side to side-speeding up the air that way.
Narrowing side to side is another way to increase the air velocity by decreasing the cross section in the tunnel. The inverted wing works just like an airplane wing, only - inverted. Any means of speeding up the air under a car can increase downforce. Most modern car designs use both shapes in the tunnels - narrowing side to side, and using an inverted curve shape in the top of the tunnel, the road being the bottom of the tunnel.
Is there one basic design? Or do some tunnels simply have an inverted wing design while others narrow?
I'm sure there is no basic design, but any design used must be within the constraints of the car's design, physical laws and the racing rules (most of the time).
Hope this sheds some light on it for you.
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