https://www.cdc.gov/oral-health/about/about-dental-fluorosis.html https://www.cdc.gov/oral-health/about/about-dental-fluorosis... claims consumed fluoride is us

https://www.cdc.gov/oral-health/about/about-dental-fluorosis.html https://www.cdc.gov/oral-health/about/about-dental-fluorosis... claims consumed fluoride is used when first growing teeth, which occurs beneath the gums and where no topical fluoride is applied. This also seems to be how fluorosis occurs, from excessive consumption of fluoride, with too much being digested and too much ending up in tooth enamel. But further research points to https://www.adaq.org.au/Web/Web/Profession/Fluoridation_FAQs.aspx https://www.adaq.org.au/Web/Web/Profession/Fluoridation_FAQs... , where item 4 indicates that, while true that fluoride is used when enamel is first formed, that it does little regarding cavities as the vast bulk of the work is done topically as you say, after the tooth erupts. I can't find anything stating fluoride is required by children to form tooth enamel, or what happens when fluoride is topically applied but somehow not swallowed.