For example, you can download Sierra and download High Sierra from its site. In case your Purchased tab doesn't contain the version you're looking for, you might be able to get it via the Apple website. Just go to your Purchased tab and you should see them listed among your other apps. If your Mac is running a version of macOS older than Mojave, you can download many versions through the App Store. There are several different ways to get a copy of macOS to install on your USB drive.
