Whether you can rank for a specific thing people are searching and looking for is also a good litmus test of 1) is there demand for your thing ... are there people searching Google for your solution, and 2) is the market not so extremely saturated with competitors that you're able to rank?
I run a translation company, so when we started the target was not "website translation" .. it was "website translation for squarespace" (and similar niche use cases which our product worked equally well for). A the company grew, so did the breadth of our use cases.
Our first 500 users were people paying $10/mo to translate their squarespace site. The next 500 users were enterprise companies paying many orders of magnitude more to translate everything/anything you can imagine.
And as for "the first" customer... that would be the company I was working for while developing the MVP. Who actually paid me $20/mo for the product while I was working for them! (And yes, the employment contract said they owned all my IP for side projects, and no it wasn't difficult to get them to sign a simple letter negating those terms since I was open and honest about my side projects - people on HN go bonkers with legalities of contracts when in reality people tend to operate on good faith)
The 2nd customer was a cold outreach where I offered to basically do all of the work manually if the solution I was offering didn't work off the shelf. Essentially I was offering them free professional services.
The 3-10th customer came through the SEO scheme.