Rob Walling has written and spoken a lot about marketing for developers. Rob published a book in 2010 titled "start small stay small" https://startsmall.com/ . A lot of the specific tricks and tools may be a decade out of date now, but some of the ideas wont -- one of them is not to build a thing that people aren't looking for, but find a niche that people are already looking for (e.g. tapping into search traffic for related keywords and so on) and run experiments to see how much traffic you can get to a landing page / sign up page for a product to serve that niche, to evaluate if there's enough demand for a product, before you invest in building anything.
Another aspect to this is -- don't focus on the product/service/technical project, put that aside. What are some interesting niche communities of potential customers? why are these niches of customers interesting, from a business perspective? How hard is it to sell to them? Will there be a lot of competition? Are there enough of them with a similar need or jobs to be done that you have a chance of supporting a business? what methods do you have to communicate with them? if you build a product or service for them, how will they find out about it? If you think a bit about Rob's products and services linked above, it's broadly clear which market niches of potential customers he is selling too.
More recently, see also https://microconf.com
Another aspect to this is -- don't focus on the product/service/technical project, put that aside. What are some interesting niche communities of potential customers? why are these niches of customers interesting, from a business perspective? How hard is it to sell to them? Will there be a lot of competition? Are there enough of them with a similar need or jobs to be done that you have a chance of supporting a business? what methods do you have to communicate with them? if you build a product or service for them, how will they find out about it? If you think a bit about Rob's products and services linked above, it's broadly clear which market niches of potential customers he is selling too.