Adam and Eve did not only have sons. They also had daughters. The Bible doesn’t mention them until after it talks about Seth (Genesis 5:4). It’s most likely they had sons and daughters at the same time that Cain and Abel are mentioned.
The Bible doesn’t mention women, or daughters, very much in the genealogies. Matthew is one of the only ones to mention women regularly, four of them in his opening genealogy of Jesus. Every time men and women are mentioned in genealogies, it is for a special purpose by the author.
So women and daughters weren’t mentioned in the genealogies very much in Genesis because it focused on the paternal side. It referenced the males because they were probably recorded in the records Moses referenced when he wrote genealogies in Genesis.
Unfortunately, women are mentioned as an exception rather than the rule. This doesn’t mean they weren’t important to God’s plans. In fact, many women played a vital role in moving forward his salvation plan.
Just as some examples, Matthew mentions Rahab, who was instrumental in the line of David, from which Jesus came. Esther saved the entire nation of Israel during occupation. Ruth was a grandmother to David, even though she was a foreigner.
Mary, Jesus’ mother, perhaps was the most instrumental of all. Many of these women experienced God’s supernatural healing and empowerment to play their part. Although they are not mentioned along with many of the men in the Bible in these genealogies, they were very important to God.
The reason Cain can find a wife and there are many people on the earth already is that Adam lived for 930 years. That’s a long time to have children. We don’t know how old Eve was when she died, but she was instrumental in helping to populate the earth.