What are the most cost-effective channels while promoting your digital product?

What are the most effective backlink and outreach channels you use to attract more users to your product apart from paid channels for the US market? I have already used: -Reddit -Slack communities -quora (I'm not so sure about the quality traffic) -blog content creation and distribution (you can suggest any dist. channels btw) -Twitter -Forums for the developer team lead, project manager, and founder personas