Cavities also are known as caries develop on teeth mostly due to plaque buildup in the mouth. Enamel is the outer layer on a tooth and it is the protective layer to prevent caries. However, with a plaque build-up, which is a collection of bacteria, acids are produced and the enamel layer is demineralized causing caries developing in the tooth. Once caries develops, it becomes over time and without a dental treatment such as filling, it can grow to a significant size, where either root canal or extraction may be needed. Therefore, it is the best practice to take care of caries when they are small. Fillings have a better prognosis when they are treated when they are small in terms of post-op sensitivity, discomfort, etc. Caries is an irreversible process and it needs to be treated.

These are things that can prevent cavities

Good Oral Hygiene
periodic regular cleaning by your dentist. Even you may be having good oral hygiene, people have missing spots where plagues are building up without knowing it.
Sealants – by putting sealants and closing the pit and fissure on top of molar it can prevent caries to start forming. (Please look at the article about Sealant: [https://www.midentalclinic.com/what-is-sealant/])
fluoride – fluoride prevents enamel layer from being demineralized and also helps with remineralizing the layer by replenishing calcium and phosphorus. (please look at the article about fluoride: [ https://www.midentalclinic.com/fluoride-treatments/ ]
non-frequent snacking. Frequent eating can cause caries. Each time we snack or eat, oral pH level (due to acids) goes down causing demineralization and after 20-30 minutes the pH level goes recovers. More frequent exposure to the low pH level is known for causing caries
Healthy diet avoiding sugary snacks, foods, drinks, etc
Avoid smoking; it can cause dry mouth

Also, if you have a medical condition such as Sjogren’s Syndrome, Diabetes, or etc, it can make your mouth drier causing caries; Salvia contains minerals that can protect from caries but with the lack of saliva, it can cause more carries.

Generally speaking, if caries is not small enough, you can get fillings but if you leave caries for a long time, it can grow big enough so you may need a root canal or extractions. Therefore every 6 months exams are an essential part of complete dental care.

References:
https://en.wikipedia.org/wiki/Tooth_decay
https://www.ada.org/en/member-center/oral-health-topics/caries-risk-assessment-and-management
https://www.mouthhealthy.org/en/az-topics/d/decay