Especially the really small ones on the side.
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There are 3 major technologies: optical, ultrasonic and capacitive.
This is an incredibly huge simplification, but opticals basically take a picture of your fingerprint, but of course the sensor is modified so that it’s not actually a picture and better suited for fingerprints, detecting the pattern better.
Ultrasonic fingerprint sensors send mechanical waves to your finger, the waves bounce back in a way that’s unique to each finger print, and then the sensor proceeds to detect and analyze the mechanical waves. If you wanna try it for yourself, place your hand on the side of a solid object, and slap the other side of the object (not too hard, don’t break anything!). If your first hand felt a vibration, congratulations, you just sent and received a mechanical wave!
Finally, capacitative sensors are like mine fields. The sensor has tons of tiny little “mines” that when the sensor is turned on, any part of skin that is currently on a mine has that mine blow up, and those that don’t have their mines intact. This then allows for the image of the fingerprint to be recreated from what mines are blown up and which aren’t.
In this analogy, the mines can detect skin because skin can somewhat conduct electricity, and without getting to complicated, each individual mine checks if electricity can flow (because skin is conductive) and blows up, while those that don’t blow up couldn’t conduct electricity because they were touching air instead of skin.
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