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Where these things run. Installed software and web products, separated — because 'a Windows app' and 'a website' are different searches.
Runs in a browser, nothing to install. The default for most indie products, which also makes it the most crowded place to be.
Native Mac software. Historically where indie desktop development concentrated, and where users are most used to paying for it.
Desktop software for Windows. A market plenty of indie developers skip in favour of web and macOS — which is exactly why there's still room in it.
Android apps built independently, outside the big publishers.
iPhone and iPad apps from independent developers and very small teams.
Software that runs on Linux desktops and servers — often where developer tools land first.