The common recommendation I've seen is to use your network. If it's something you're building then ideally you know what problem you're solving and who has that problem. Then getting those early users you know with that problem involved to test and give feedback, you can then optimize further for that audience and need. From there those users will likely share with those that they know within their own network having the same issue. There's a few books on it like Tribes by Godin (though not everyone agrees of course).

Other than that there are a few places that highlight product launches like product hunt, etc. to gain initial usage. Probably check for niche groups having the issue and reach out organically - no one likes spam.

This is just what I've gathered from others, I'm in the beginning phase of this myself :-)

Often solo-founders build very niche apps/SaaS which they already have a personal network for or can target smaller platforms and business networks for marketing/distribution. You can stay small and do very well.

The larger platforms are usually too saturated to be effective. In my experience, generally not a good return on $/time.