Where to download older OS version from App Store?

You can not go backwards by just downloading and installing an older macOS version.

You must first completely wipe the Mac using macOS Recovery Mode,

as per > https://support.apple.com/en-us/HT208496

Then reinstall the macOS version that it came with using the "Other macOS installations options" instructions

at > https://support.apple.com/en-us/HT204904

Other macOS installations options

When you install macOS from Recovery, you get the current version of the most recently installed macOS, with some exceptions:

• On an Intel-based Mac, if you use Shift-Option-Command-R during startup, you're offered the macOS that came with your Mac, or the closest version still available. If you use Option-Command-R during startup, in most cases you're offered the latest macOS that is compatible with your Mac. Otherwise you're offered the macOS that came with your Mac, or the closest version still available.

Hi Vincent what are you trying to do it’s a downgrade , to get to a previous macOS isn’t just like that , to do all that procedure you will lose everything that you got in that Mac I can guide you step by step to get that my questions for you , are you aware that you can’t just install an old macOS version from a link where you can download the macOS you want without wipe out the hdd of that Mac ? If you make a backup with a new macOS then you want to restore it from old macOS won’t work you know that ? If yes then reply to this message soon and I will guide you how to do the downgrade

try here. https://mrmacintosh.com/10-14-6-supplemental-update-safari-14-released-to-fix-previous-issues/