How did people come to earth?
If you are asking how WE came to earth, we know that our Heavenly Father (the father of our spirits) set things up such that we get here through the process of birth.
If you are asking how Adam and Eve came here, as the first people to live on the earth, Heavenly Father made the bodies that housed the spirits of Adam and Eve.
As with many parts of Heavenly Father’s plan, we have not been given the details. We are here, in part, to see how well we can follow the plan given to us. We have access to all the things we need to know, to complete our probation here.
In some ways, this life is like the final tests in school; we were taught the important things needed to pass the class. While there are exceptions, most classes did not allow us to have our books and notes with us during the big test. It is necessary for us to develop faith: the ability to follow rules, even when we may not have all the information we want.
Summary: In this life, there are many things we apparently do not need to know. But, when we leave this life, we will have ALL information available to us, and the time to learn, etc.