Some additional sources:
U.S. Forest Service --
https://www.fs.usda.gov
AllTrails --
https://www.alltrails.com/explore?ref=header
TrailForks, by region --
https://www.trailforks.com
Mountain biking routes --
https://www.mtbproject.com
Trek Bikes, in their "greatrides" section --
https://www.trekbikes.com/us/en_US/greatrides
GravelMap --
https://gravelmap.com/search
Your local and regional biking associations, parks & recreation departments, and local bike shops. Many have references to great trails and routes in the area, and some will have maps. Most of the major metro areas, in the U.S., have produced maps of the bike lanes, multi-use paths and bikeable through-ways in and around the city.