Single track info:. https://www.michigan.gov/dnr/things-to-do/orv-riding/where
That interactive map is a great reference.
This site has a lot of great info, even if a little dated. They have descriptions of a lot of the trails.
Trail Information - VVMapping.com
GPS, PDF and Printed maps of ORV Trails and Snowmobile trails in Michigan.
www.vvmapping.com