For us, it's pretty simple to talk about our product because we're helping improve sleep. It's such a common problem in today's society, and the existing "solutions", focus on comfort, other physiological changes (cool pads, weighted blankets), ear plugs, eye masks, etc etc. We're using neuro-feedback to alter your sleep state. It's pretty magical. If you want to find out more, check out https://soundmind.co
So, we talk to people and not only about their sleep, but about how it affects their life, what do they do about it, what do they try, what works for them, what doesn't. From here, we were able to build comprehensive user personas, and those personas describe where our users are.
It was also super interesting for us to find that almost nobody talks to their doctors about sleep, very few people we spoke to had been to a sleep lab+.
When you say there are 100s of target segments, yes, you're right, but you don't care about those. I think you care about two things.
1) can you create a customer persona or two, which are large enough to be viable to get the business started
2) is there a common thing that they do, or place where they congregate to reach them?
Don't make a checklist of all the places they could be or all the possible segments, just talk to people, and look for the commonality, let them self-suggest.
That's my 2 cents anyway.