Great question! I have entire, semester-long classes about about this exact issue. The basic gist is that entrepreneurs shouldn’t start with a product/service (i.e. an “idea”). They should start by identifying and validating a market opportunity.
I explain what I mean by a market opportunity in this article: https://medium.com/swlh/can-you-avoid-the-one-mistake-every-entrepreneur-makes-29c918347d4d
(The TL:DR is that a market opportunity is significant demand + cost effective access to that demand.)
Once you’ve proven a market opportunity, then you can start figuring out the best way to solve it (e.g. developing a product or service). By that point, you should know who your audience is because your audience consists of the people creating the significant demand.
In case you’re interested, here’s an article where I explain all of this in more detail: