There is a download link here to get the Big Sur installer..
Or I should say it Says that's what it is!!
• macOS Big Sur 11 can upgrade Catalina, Mojave, High Sierra, Sierra, El Capitan, Yosemite, Mavericks
The blue link ^ goes to: https://apps.apple.com/us/app/macos-big-sur/id1526878132?mt=12
These are located in Support article:
• How to get old versions of macOS - Apple Support
https://support.apple.com/en-us/HT211683
Additional info and instructions are within above.
Most of these upgrades are like a one-way ticket; even
if you do create a great backup strategy (in triplicate.)
Your issue may be relocated to form new separate thread.
Good luck & happy trails!🌤🌿🌻❄️