I don't know what they do in the Netherlands, but in rural parts of the United States where people are on well water, fluoride treatments are common. Florida is directly applied to the teeth during a dental visit, then washed out.
Anecdotally, this is what I had and I have great Dental Health despite for brushing hygiene. I often skip brushing my teeth and almost never floss and haven't had a cavity in 40 years
Fwiw, I’ve pretty much always lived with access to municipal water with added flouride and have also always been given flouride treatment at the dentist. I think the latter is pretty universal in the US.
Anecdotally, this is what I had and I have great Dental Health despite for brushing hygiene. I often skip brushing my teeth and almost never floss and haven't had a cavity in 40 years