Many people think the shape of a tree’s root system mirrors the trunk and crown but in reality, most roots are in the top 12” of soil. Tree roots anchor a tree, keeping the tree standing straight plus taking needed nutrients and water from the soil. Most of a tree’s root system is close to the surface though many also have a deep tap root. When trees are transplanted, some of the root system is left behind, so don’t prune before or right after re-planting. That weakens the tree’s ability to get established in the new setting.
The majority of roots are in the top 12” of soil.
For more information, see the Purdue University Extension bulletin on tree roots that surface: https://extension.purdue.edu/Gibson/pages/article.aspx?intItemID=12124