How do I downgrade from Big Sur to El Capitan?

Why did you upgrade then?

And do you really need to go as far back as Mac OS X El Capitan,

El Capitan is no longer supported by Apple and does not receive security updates.

First you need to make a full backup of your mac.

Second you need to download the Mac OS X El Capitan installer disk image.

Click on the link below,

How to get old versions of macOS – Apple Support

Go to Download OS and click on OS X El Capitan 10.11

this downloads InstallMacOSX.dmg to your Downloads folder.

The next section can only be done on a mac that is capable of running El Capitan.

This includes macs that have the potential to run El Capitan but have been upgraded to a newer OS.

A mac that came preinstalled with an OS later than El Capitan will refuse to do the next bit.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install El Capitan

but converts the InstallMacOSX.pkg to the Install OS X El Capitan.app which 

you will find in your Applications folder, it should be 6.2GBs in size.

(If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install El Capitan on, that is for later.)

Create a bootable USB installer disk using the Install OS X El Capitan.app in the Applications 

folder and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support

This bootable USB stick needs to be plugged in directly to your mac.

Restart the mac while pressing and holding down the option/ alt key.

In a couple of minutes you will see the Startup Manager, select the USB

and press Return.

When the USB has booted up you will see a Utilities panel select Disk Utility

and press Continue.

As a rule any OS from El Capitan or earlier cannot see disks that have been formatted

as APFS which Big Sur works on so you will not see the full Disk, Volumes and Containers tree

you may just see the Disk

If you do highlight it and click Erase.

Give the Disk a name,

Format: Mac OS Extended (Journaled)

Scheme: GUID Partition Map

Click Erase.

When Done quit Disk Utility.

Click on Install OS X press Continue.

The installation process should now startup follow the prompts.

When the installation is completed your mac will boot up to the internal drive

running Mac OS X El Capitan version 10.11.6.

Set up your mac, open the App Store and install any updates that are available

in the Updates section.

After setting the mac up and updating it then you can restore your personal information from the

backup you made earlier.

Do not restore any system or library files or any apps. Only restore items from your Users/<Home> folder

such as Movies, Images, Documents etc. As said any items you may have created or edited while running

Big Sur may not be backwards compatible with the equivalent apps that run in El Capitan.

If you used the Photos app in Big Sur the photoslibrary where your images are stored is not compatible

with the version of Photos on El Capitan and cannot be retro migrated to work with El Capitan.

This may be the same with the Music app in Big Sur and iTunes in El Capitan.

I am going to boot my mac back into El Capitan and run Disk Utility just to make sure it can access and

erase an APFS formatted drive, I will get back to you.

Really you want to do that, why.

This would involve you downloading the Mac OS X El Capitan disk image

processing that to get the Install OS X El Captan.app, then creating a bootable USB installer.

Then you have to back up your mac.

Then erase it completely.

Use the bootable USB installer to install El Capitan.

Restore only your personal data from the back up you make.

Any files you may have created or edited in Big Sur may not be backwards

compatible with the versions of apps in El Capitan.

Sure you want to go ahead, if you do I can give you more complete instructions.

Ok so opening Disk Utility in El Capitan the APFS formatted drives are only recognised as

generic APFS Media.

When you open Disk Utility from the bootable USB you will probably only see the Disk

probably called Apple SSD… and the containers and volumes of Big Sur will be merged

into one indented volume called APFS Media or similar. It is the Disk you need to highlight and then click Erase.

follow the instructions listed in my earlier post.