Dirt dauber wasp nest on the alternator?

Typical of mud wasps. Vehicle sets for 5 minutes, and they decide it's home. Not much deters them. If you can, park it in a different spot and see if they return. Rodents and bugs always seem to think cars make good homes.

The cowl is basically an open space under the cover. Usually has things like the air inlet to the heater, wiper motor and transmission linkage. Bugs can build nests pretty much anywhere they can gain access to. Most of them don't really hurt anything though, they don't chew on the wiring or plastic, they are generally just annoying. The ones to watch for are wasps and hornets that sting or bite. They like to build in the door jambs and in similar places. However, they generally take a few days to build a nest or comb so a car that is in use isn't commonly a target because they have a hard time tracking a moving nest. Not really much you can do to keep them out because they can fit through tiny gaps. The worst thing is that putting up something like a bug zapper usually makes it worse. You put it up, so it attracts the bugs and zaps them, but because it attracts them you end up with more in that area and it tends to hurt the good bugs more than the bad ones.

Yes they can get into the inlet side, but then they hit the cabin filter which means they can't get inside the car itself. Most cars have an inlet screen on the outside as well that blocks larger items. Others don't. Easy way to tell would be to replace the cabin filter. It's behind the glove box. You empty the glove box out, then remove the two stops from the holes and disconnect the damper on the outside. Then drop the box down and release the clips on the cover. Now look at the filter for the airflow direction. Then remove it. Look it over for any damage or signs of critters. Install the new one with the arrow pointing the correct direction. Install the cover, put the glove box back together. Throw out the 2-year-old tootsie rolls, and leaky pen (lol) then enjoy. FYI if you can find them, they make a carbon filled filter for many cars as well as baking soda ones. I use those rather than a stock filter as they can help if you go through areas with dubious air quality. I used to drive past a rendering plant and in the summer, it got really bad. The baking soda one helped with that.

They build wherever they find a handy spot, I've seen them in empty holes, under a component, on top of the valve cover plus multiple other areas.

They are bugs and will build wherever they want. The cowl is open in many places other than the slits you see. Can they get inside? Maybe, but is it likely they will - no. There is no way to know where to look because they can get into lot's of places that you would need to take the car apart to even find. There are also no real deterrents that actually work.

If you mean the brown stuff, that is factory seam sealer. If you were to strip the car down and pull the paint off you would see that all over the place where the various body panels are put together and they want to stop moisture from getting in the seams. That is just an area where they didn't smooth it out real well.

Nothing to worry about with that stuff. It's pretty much like caulking the gaps and cracks on your house.

They have used it since the 1940's when all steel bodies became the normal thing, but some folks are better at not making a mess with it or applying too much so it squeezes out like that. You should see cars from the late 70's through the 90's there would be globs of it and places where they forgot to place any, then other times it looked like they applied it with a 4 inch paint brush. I've looked at cars on the lot and it's amazing to see the differences on "identical" vehicles.

They seem to understand better than humans that you don't build in areas where your home gets removed by forces you can't control. But then again, the nests are made by the females. I never bother with them myself, unless they built a nest in a bad spot.