I saw this on Twitter recently - it's a statement by the Endocrine society and it sums up the broad consensus quite well and has links to some papers:
www.endocrine.org/advocacy/position-statements/transgender-health
"The medical consensus in the late 20th century was that transgender and gender incongruent individuals suffered a mental health disorder termed “gender identity disorder.” Gender identity was considered malleable and subject to external influences. Today, however, this attitude is no longer considered valid. Considerable scientific evidence has emerged demonstrating a durable biological element underlying gender identity.1,2 Individuals may make choices due to other factors in their lives, but there do not seem to be external forces that genuinely cause individuals to change gender identity."
You could use that as you starting point for further research.
This statement isn't new, @Positrans, it's from 2017. Here are some of the comments I posted a couple of years ago when someone else was trying to use it to prove that gender identity was biological:
Thie Endocrine Society statement cites 5 distinct references from journals (3 of them are cited 3 times each to bulk out the reference list).
The first reference is a very short literature review, Saraswat A, et al. Evidence Supporting the Biologic Nature of Gender Identity. Endocr Pract. 2015 Feb;21(2): 199-204.
This paper concludes: "Because the sample sizes of most studies on this subject were small, the conclusions must be interpreted with caution. Further research is required to assign specific biologic mechanisms for gender identity."
Despite this, the Endocrine Society use it to claim that "the data are strong for a biological underpinning to gender identity".