Quick tip. If you're trying to download really old versions of macOS, they're hard to find (e.g., El Capitan, etc.). You can find lots of dodgy sites that you don't want to deal with, but it's hard to find legit Apple web sites. The trick is to search for "enterprise". Then you can find pages like this, which has a legit link to an Apple DMG. https://support.apple.com/en-us/HT206886
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