How do you find demand? I will not promote

It’s easy to build something but finding what to build is the problem. How do you find genuine demand for a startup before building? How did you find yours? I’m in a pain cave for years. I tried two approaches - First one was a startup every 2 months. 6-7 failed ones. Revenue $7. 300 free users max. - Second one. Take an idea and build for a year. $700-1000 MRR stuck with 3 customers. New ones don’t stick at all. + everything done half manually (don’t want to double down if there is not enough demand) How does true demand feel? Like people find you, call you and ask give me your product I’ll pay?