As a US kid in the 2000s my dentist would give some kind of foam fluroide treatment. Foam would be put into two trays and you'd have to keep them on your teeth for two minutes. I remember hating it because it made me nervous I'd swallow some.

I still get fluoride treatments from my dentist but now it's something he paints on, perhaps the same as what you've described.

Anyway, according to a dentist friend, whether you get cavities is also dependent on the natural groves in your teeth. Some people's genetics cause them to have more cavity prone teeth than others.

Yes that's the stuff though for me it was gel in trays, not foam.

I was worried too about swallowing it but it was also the smell of it that made me sick. It was very strong. I really hated it.

And yeah I guess I'm not very prone to them, but it's very clear that the generations before me had much worse teeth than we do.

Without insurance you can often get reasonably priced dental care at dental universities if you have one local.

At least for anything students can work on (fillings, extractions, etc). If you need a root canal, ortho, jaw surgery, or etc then I doubt it'll be a great option.

I strongly urge people to avoid regular dentists for serious work that there are specialists for. They don't have the tools or expertise, and often leave people with broken jaws and infections that lead to more surgeries. Pay the extra; you'll wish you did when your jaw is wired shut, or you have to pay again to re-do a root canal, or go to emergency due to infection.