Old devices on macOS (AFTER 2011*) update on BIGSUR with OpenCore Legacy Patcher.
GUI APP (:
You need a macOS machine ->The app run on macOS system.
Direct download here 02//05//2022 version : 0.4.4
https://github.com/dortania/OpenCore-Legacy-Patcher/releases/download/0.4.4/OpenCore-Patcher-GUI.app.zip
Docs & tutorial here :
https://dortania.github.io/OpenCore-Legacy-Patcher/INSTALLER.html#creating-the-installer