Many often conflate two concepts in biology: how sex is defined versus how sex is determined. Here’s why the difference is important.

This animated video was based on a longer article at the Paradox Institute.
https://www.theparadoxinstitute.com/read/defining-sex-vs-determining-sex

Sources:
[1] Lehtonen, J., Parker, G. (2014). Gamete competition, gamete limitation, and the evolution of two sexes. Molecular Human Reproduction, 20(12).
[2] Lehtonen, J. (2017). Gamete Size. Encyclopedia of Evolutionary Psychological Science; Scharer, L. (2017). The varied ways of being male and female. Molecular Reproduction & Development, 84.
[3] Bhargava, A., et al. (2021). Considering sex as a biological variable in basic and clinical studies. Endocrine Reviews, 20(20).
[4] Holub, A., Shackelford, T. (2021). Gonochorism. Encyclopedia of Animal Cognition and Behavior. Springer.
[5] Eggers, S., Sinclair, A. (2012). Mammalian sex determination: insights from humans and mice. Chromosome Res, 20.
[6] Rey, R., Josso, N., Racine, C. (2020). Sexual differentiation. In--Endotext. South Dartmouth, MDText, Inc.
[7] Bachtrog D, Mank JE, Peichel CL, Kirkpatrick M, Otto SP, Ashman TL, et al. (2014). Sex Determination: Why So Many Ways of Doing It? PLoS Biol, 12(7).
[8] Avise, J. (2011). Two sexes in one. In: Hermaphroditism: a primer on the biology, ecology, and the evolution of dual sexuality. Columbia University Press, 5.
[9] Scharer, L. (2017). The varied ways of being male and female. Molecular Reproduction & Development, 84.
[10] Beukeboom, L, Perrin, N. (2014). The evolution of sex determination. Oxford, UK: Oxford University Press.
[11] Kimball, J. (2020). Sex chromosomes. LibreText.org; Ellis, PJ., Erickson, RP. (2017). Genetics of sex determination and differentiation. Fetal and Natal Physiology (Fifth Edition), Elsevier.
[12] Rey, R., Josso, N., Racine, C. (2020). Sexual differentiation. In: Endotext. South Dartmouth, MDText, Inc.
[13] NIH. (2021). 46,XX testicular disorder of sex development. MedlinePlus Genetics.

(From Youtube)