You’re not an idiot. The gist of it is that there have been places that didn’t have or removed fluoridation and it resulted in terrible dental health in kids.

And yes you can get fluoride from toothpaste. But I’m guessing what happened is that due to the risk of swallowing toothpaste kids need low fluoride or fluoride free toothpaste, so weren’t able to get enough without having it in the water.