I think it's like marketing any other type of software or service online - SEO, find user communities, influencers, ads/paid promotions etc etc. In my case I got lucky and impressed a tech writer who gave BrainTool a write up on ZDNet and gave me a jumpstart on the first 2K+ downloads. I wrote about the experience here:
https://braintool.org/2021/01/14/First-Two-Thousand-BrainToo...