Hello and welcome

Here are a few resources that might help understand:
http://www.acceptingdad.com/2013/08/05/to-the-unicorns-dad/http://www.gires.org.uk/assets/DOH-Assets/pdf/doh-transgender-experiences.pdfStudies by reputable sources are shown there. Its also explained with pictures.
Studies have shown there are differences in brains of women and men so a mismatch is possible.
There are even substances known to cause a higher rate of transgender people. They are off the market now.
Also transgender people have been around in all cultures so its not a cultural thing.
The brains of transgender people are wired this way before birth and there is no known way to change this.
Transgender people suffer from a condition called gender dysphoria.
It can be body related or social, like how they are perceived, preferring certain pronouns etc.
So its possible they do not like some parts of their body.
You might talk about it.
The remedy is often hrt and transition, depending on what the transgender person feels they need. Most people are much more happy afterwards.
Many transgender people have the support of a gender therapist to help them along.
You might visit there once or twice if your partner has a therapist so you can get a better understanding of the individual dysphoria of your partner.
I'd say just talk to each other ...
and you can ask questions here ...
there is also a chat on susans...
and of course your partner is invited too...

hugs