Answers here are focused on proteins and that is correct, but nucleotides (DNA and more abundant RNA) are also an important source of nitrogen in urine. In bacteria, there is about half as much nucleic acid by dry weight as protein. Nucleotides are less nitrogen-dense than proteins but still a major contributor to nitrogen in urine.

Lots of coevolution. Think about how dependent animals are on plants - not just for food but for oxygen. And plants have coevolved with Cyanobacteria symbionts to the point where we don’t even recognize chloroplasts as independent organisms…jammed into the cell with another co-evolved endosymbiont, the mitochondrIon. Co-evolution with bacterial microbiome partners provides help preventing infections in animals, and bacteria in the digestive tracts of ruminants allows them to digest cellulose.