I tried many years ago but couldn’t readily find an explanation for the process/reasoning, but I did find one now in the video here and it does make sense I also checked Wikipedia and learned more:

All fluoridation methods, including water fluoridation, create low levels of fluoride ions in saliva and plaque fluid, thus exerting a topical or surface effect. A person living in an area with fluoridated water may experience rises of fluoride concentration in saliva to about 0.04 mg/L several times during a day.[3] Technically, this fluoride does not prevent cavities but rather controls the rate at which they develop.

Fluoride’s effects depend on the total daily intake of fluoride from all sources.[51] About 70–90% of ingested fluoride is absorbed into the blood, where it distributes throughout the body. In infants 80–90% of absorbed fluoride is retained, with the rest excreted, mostly via urine; in adults about 60% is retained. About 99% of retained fluoride is stored in bone, teeth, and other calcium-rich areas, where excess quantities can cause fluorosis.[79] Drinking water is typically the largest source of fluoride.