where can i download big sur? cant download from apple

i made a usb with sonoma and it didnt work on my macbook air (mid-2012) so im looking to go in increments. i need Big Sur because im currently on Catalina .

using OpenCore it keeps failing to download so i tried it on a mac mini m2 through app store and it says it is no longer available. i looked all over the internet and the downloads do not work.

if anyone can point me in the right direction to get this old mac up to date i would appreciate it

Author: @Charming_Ad9373