How do you figure out what products to make a business around?

>>60616356 (OP)
tailored underwear for busty teenagers, prevents breast pain during growth phases
ballerina style shoes for growing teenage girls, prevents foot damage and callus buildup
you gotta test that extensively

By doing market research.
>inb4 how to do market research
By researching how to do market research (on google).
No one that is good at this is going to fully disclose their secret sauce.