As a long time Betta owner, this is a personal pet peeve of mine. Yes, they CAN live in small tanks. No, they should NOT live in those tiny tanks that pet shops sell for them. It actually causes them quite a bit of stress and Bettas that live in those tiny tanks have a decreased life expectancy than Bettas given more space (often by a few years). They like larger tanks with swimming room. They are also not as “easy” as many people assume - they are quite fussy about water temperature (and generally need a good heater), the water quality needs to be monitored, and their diet can also be a bit tricky (they are prone to swim bladder and other similar disorders).
They are wonderful fish, but they are not the “easy fish that only need a tiny tank and minimal care” fish that some pet stores make them out to be. At least not if you want them to live to their full life expectancy potential.