When you are a indie dev, and you dont plan to get a publisher, i would say create like a small dev log on Twitter or itch io, let people follow your work and let them see how the game goes from 0 to hero, this way you got a base audience when your finished.
Putting your demo project in Kickstarter etc is a good way aswell.
But honestly, i work with web development and i made like 100s of websites like web shops to customers etc, and i tell you what i tell them, advertising costs money, and a lot of people are pumping money into advertising , so its kind of hard to compete as indie dev, thats why creating a dev log is a nice and free way to get a core audience. Or finding a publisher that does this for you ( dont be afraid most publishers will take your game and publish it , no matter how non significant you think it is, because it can make money )
Also, dont forget , your game will advertise itself if its a really good game 😉