1. Find the installers
Apple’s download page explains that a full installer from the App Store lands in your Applications folder as an app named Install macOS followed by the version name. Open Applications and sort by size; if it is there, it is usually the largest single item. Storage settings lists it under Applications too.
Disk images are the second source. Look in Downloads for .dmg, .pkg and .zip files. Each is a copy of an installer you already ran, and the app it installed lives in Applications, not inside the image.
2. Delete the macOS installer
Once the upgrade has finished, drag Install macOS to the Trash. Apple provides its installers for download again, so keeping it is a bandwidth trade, not a safety one. The exception is a bootable installer: Apple’s guide for creating one requires the full installer in Applications, so keep it if you plan to make an installer drive for another Mac.
If macOS says the installer cannot be deleted because it is in use, the Installer app is still running or a volume it mounted is still attached. Quit Installer, eject anything it mounted from the Finder sidebar, and try again; if that fails, restart and delete it before opening anything else.
3. Eject disk images, then delete them
A .dmg that is open appears as a volume in the Finder sidebar. Click the eject icon next to it first; a mounted image counts as in use and blocks deletion. Then move the .dmg in Downloads to the Trash. The installed app keeps working, because it was copied out of the image when you dragged it to Applications.
Package files (.pkg) work the same way: the installation is complete, and the package is a leftover. Keep only the ones a vendor asks you to keep, which is rare.
4. Empty the Trash and check
Apple’s storage guidance is explicit that a file’s space is not available until you empty the Trash. Empty it, then compare System Settings → General → Storage with the figure you started from. A single installer often returns more space than an afternoon in the Library.
When the Trash refuses, the Trash guide covers locked items, files in use and Delete Immediately.
5. Keep it from piling up
Two habits prevent the next pile. Use Software Update rather than a full installer for routine upgrades; Apple notes it can use less storage to download and install. And clear Downloads after each install, or set your browser to ask where to save so installers stop landing there by default.
A free local scan lists disk images and installer apps by allocated size wherever they ended up, including a second copy in a Desktop folder or an external drive. ClearDisk shows them, lets you reveal each in Finder, and moves what you choose to the Trash first.
References & further reading
These guides combine the linked official guidance with ClearDisk’s documented behavior. Your files and app settings determine the right next step.