It’s a great question and you’re exactly right! There are even some cases of fish having multiple sex chromosome systems interacting together to be XYZ. Cichlid fish in particular, well known for rapid speciation, show incredible evolution of sex chromosomes. In which case, you’re unlikely to find a single degenerate Y chromosome like you find in mammals.

Temperature dependent sex determination is fascinating! Just remember, mammals and birds use chromosomal sex determination (meaning sex is genetic) but other mechanisms are quite common across all organisms! Check out the cool graphs at tree of sex http://treeofsex.org/.

There is still a lot of work left to be done on mechanisms of how temperature during incubation leads to different sex ratios. This is a good scientific review on red-eared sliders https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2695493/. The ultimate mechanisms, or why temperature sex determination occurs, is also an interesting evolutionary question. Here is a good starting point for further research: https://sites.utexas.edu/crewslab/files/2016/06/CrewsDVLGen.pdf.

Finally, it’s important to remember that sex is a complex phenotype with many components and is really a shorthand for biologists to describe a common suite of traits. But thanks to sweet, sweet variation there is no one size fits all! In fact, it’s not uncommon for sex to be broken down into at least 5 levels of animal sexuality but that’s just the beginning. Not to mention human-specific levels, such as gender. Just a fun aside, the genderbread person is a nice primer on gender https://www.genderbread.org/