Here is the critical logic you need to understand:

This specific version is often sought by users running or older hardware that cannot upgrade to the latest macOS (currently requiring macOS 14.6 or later for the newest iMovie). Important Warnings

page, fingers flying. He found the update DMG files, but they were just patches, not the full installer. The panic started to set in—until he remembered the "Purchased" tab trick. He logged into an old Apple ID he hadn't used since college, prayed to the silicon gods, and clicked. There it was. A ghost from 2019.

If you have an external SSD, you can install macOS Mojave on that drive. Reboot your Mac holding the Option key, boot into Mojave from the external drive, log into the App Store, and download version 10.1.12 to that drive.