Which volume to erase and format in Big Sur ?

To do a full erase of your Disk you need to erase the Disk.

The Disk being Apple SSD AP0512Q Media..., everything else below is termed as Volumes.

Have you made a bootable USB flash drive installer using the Install macOS Big Sur.app

and Terminal using these instructions.

How to create a bootable installer for macOS – Apple Support (UK)

After creating the bootable USB you would boot to it by Restarting the mac while pressing and

holding down the option/ alt key. In a couple of minutes you will see the Startup/ Boot Manager.

Select the USB and press Return.

From the Utilities window select Disk Utility and press Continue.

Click View in the menubar and select Show All Devices.

Highlight Apple SSD AP051Q Media.

Click Erase.

Give the Disk a name.

Format: APFS

Scheme: GUID Partition Map

Click Erase.

When Done quit Disk Utility.

Click on Install OS, press Continue.

The installation should start up follow the instructions.

To perform this action will require booting from a Bootable Installer. This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Once that is done read on for preparing the Destination computer  >> Special Notation - works on Intel Based Apple Computers & on Apple Silicon M1 CPU

Extra Special Notation regarding the Touch ID equipped Apple Computer. About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting 

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives.

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. 

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

In the Startup Security Utility - " No Security " and " Allow booting from External Drive "

User wrote " erasing everything " The difference is a 100 % Wipe versus the method from suggested by pervious Poster @Eau Rouge.

Either method will eventually arrive at the same end point

You have to Erase the Disk to remove all Volumes and completely reformat the Disk.

If you were to erase the Gokce Volume (which is the Macintosh HD Volume and therefore contains the Big Sur System)

and then reinstall Big Sur to that partition you would then create more partitions, Macintosh HD, Macintosh HD - Data, another Container Disk etc, leaving your macs Volume tree a mess.

And you would still have the original Gokce - Data Volume also, just taking up space.

Having the 2 Data Volumes could lead to confusion and problems later on

As for the Startup Security Utility I would have everything switched off but that is my preference.

Agree with @Eau Rouge in the Startup Security Utility - switched off in Normal Usages.

But - if attempting the Wipe and Install from Bootable External Installer Drive - that aspect Must be enabled. Otherwise - the Bootable Installer Drive - just will boot to the external drive at all - not way around that.

Apple - by default Disables this feature and is intended to Protect the computer from unauthorized changes being made or to circumvent Security Settings.