What apps, websites, strategies, et cetera, do you guys use to find good campgrounds?

I’ve been looking into trying a short camping trip just sleeping in my car, but I’ve found it difficult to find good campgrounds through a couple of apps I’ve tried (I haven’t put that much effort in however)

Mainly curious how people with actual experience go about it before I start really looking on my own.

I live in the southern United States to be clear.