What exactly did Cain get married to?

Scripture doesnt comment. It would have been a sister or perhaps a niece -- or granddaughter of Adam

The fact that Cain was scared for his own life after he killed Abel (Genesis 4:14) indicates that there were likely many other children and perhaps even grandchildren of Adam and Eve already living at that time. Cain's wife (Genesis 4:17) was a daughter or granddaughter of Adam and Eve.

http://www.gotquestions.org/Cains-wife.html

Chapter 6

Cain’s Wife—Who Was She?

by Ken Ham on September 13, 2007

https://answersingenesis.org/bible-characters/cain...

He married a sister, or possibly a niece - a human female. Adam and Eve's family is the only human family on earth at the time. They only recently departed from perfection, so marrying a close relative was not as dangerous as now.

Cain took a wife from the 6th day creation of men and women spoken of in Gen 1;26. Adam and Eve were created after God rested on the 7th day to be the seed line of Christ. The 6th day creation was all the races of man according to their placement on Earth. Black skin in the scorching sun of Africa, Slanted eyes for the sand storms of Asia, and so on. God looked and He was happy with His creations.

With Abel

He married his own twin sister, but lusted for the more beautiful sister of Abel. He got jealous and killed Abel for that.

He got married to yer mum

QED

Abel

Adam and Eve lived hundreds of years .. likely had scores of children, that had children, that had children .. 'one of those' likely ..

One of the people who were created in Chapter One.

Probably his sister or a niece.