Once you've found the download manager, you can pause or cancel the download. This will stop the app from downloading, and you can breathe a sigh of relief. But, if you're not careful, the download might start again automatically.
To prevent this from happening, you need to identify the source of the download. Is it a website, an app store, or a third-party service? Once you know where the download is coming from, you can take steps to stop it.