I look to the less-popular areas... state parks, national forests, other management agencies. Here in Florida the water management districts have a lot of land, and outside of hunting season most of it is seldom visited. In Oregon, the wilderness areas in the eastern part of the state seem to get relatively little traffic compared to the western half. Of course, it takes a bit longer to get there from Portland, and you don't get views of Mount Hood!
One trick I learned years ago was to leave on Friday afternoon and hike in a few hours that evening... sometimes you can get far enough ahead of the crowd that you won't see anyone else until you hike out on Sunday! And, of course, hiking on weekdays helps, if you're able to do that.
Bottom line is, especially near major cities, it can be a challenge to find anything approaching solitude. We should probably consider restrictions on numbers of people in a lot of places... camping and even hiking permits, etc.