>>718455910
They fund the game in the first place and theoretically advertise it too. Some indie devs can make games in their spare time but getting a team and making it from scratch requires a fuckload of time and money. One example of how to avoid a publisher is how factorio did it.

Factorio did crowdfunding so they could hire some actual decent artists since the original art was ass. It helped that even old factorio was very playable despite being unfinished. It was a solid foundation that they could build on up to the end.
Over the years they made massive improvements that got them more attention which led to more sales from their own website.
2014
https://www.youtube.com/watch?v=9yDZM0diiYc
2016 (steam early access starts)
https://www.youtube.com/watch?v=DR01YdFtWFI
2020 (1.0)
https://www.youtube.com/watch?v=J8SBp4SyvLc

Now they have 30+ team members and are working on some new project and can coast on factorio sales for years.