In some sense this would be recuperating energy rather than generating new energy because we use energy to pump water to your toilet in the first place. So it’s not really a power source on its own (at least in most places), it’s more akin to regenerative braking where you can capture some of the energy you spent before.

If this was something that you were going to do it would not make sense to have small turbines in every toilet. Likely the energy used to manufacture every turbine and ship it would dwarf the energy output.

https://what-if.xkcd.com/91/

Not only that but maintenance would be a big concern! Millions of devices with moving parts subject to solid waste would be hard to keep running (and there’s a good chance it would plug your toilet more often). If this were a valuable source of power you would want to do it downstream and use a single larger turbine for harvesting energy from many houses. This would likely be more efficient as the larger turbine would likely have less friction proportionally, and there would only be one point for maintenance.

You can of course collect energy from lots of sources to generate small amounts of electricity but generators are expensive devices to produce and the amount of energy you would get out of these in the device’s lifetime would not offset that initial energy investment! That’s where things get really tricky.

Your initial idea of putting a wind turbine on a car is potentially something that can work, but with an important caveat! You won’t get more energy than you put into the system, and if the wind turbine is generating energy it’s taking it away from the car’s kinetic energy. It would basically be a form of regenerative braking where you would use drag to slow the car down but recuperate some of the kinetic energy from the car for use later. Regenerative braking on an electric car will be more efficient for this because wind turbines can’t capture energy as efficiently as an electric generator directly connected to the spinning axels, and the other factor that’s a huge win for normal regenerative braking in electric vehicles is that you don’t need many additional parts for the car. The wind turbine idea means you have to build a wind turbine, but you know what’s awesome? When you spin an electric motor it becomes an electrical generator instead, so you can just use the electric motor you already have for it!

Also for what it’s worth… The wind turbine for regenerative braking mind not be a thing, but in Formula 1 they have used fly wheels to store kinetic energy instead! So basically instead of spinning a fan, they have a big heavy wheel that they can use the momentum of the car to spin up (while slowing down the car at the same time), and this wheel can later be used to speed up the car again later.

https://en.m.wikipedia.org/wiki/Kinetic_energy_recovery_system

With all of these regenerative systems it’s important to note that you don’t get more energy than you put in. There are losses and due to conservation of energy you’ll never be able to recover more energy.

That said wind power is also a thing and there’s all sorts of cool ways to take advantage of that… Like sail boats, and these cool things!

https://en.m.wikipedia.org/wiki/Rotor_ship