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How to erase an Intel-based Mac

Erase (format) built-in startup disk of an Intel-based Mac.

These steps apply only to Mac computers with an Intel processor. The steps to erase a Mac with Apple silicon are different.

Before erasing your Mac

• Make a backup of any files that you want to keep. Erasing your Mac permanently deletes its files. 
• If you want to restore your Mac to factory settings, such as to prepare it for a new owner, first learn what to do before you sell, give away, or trade in your Mac. Then erase your Mac as the final step.

Use Disk Utility to erase your Mac

1. Start up from macOS Recovery: Turn on your Mac, then immediately press and hold these two keys until you see an Apple logo or other image: Command (⌘) and R. 
2. If asked, select a user you know the password for, then enter their administrator password. 
3. From the utilities window, select Disk Utility and click Continue.
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6. Select Macintosh HD in the sidebar of Disk Utility. Don't see Macintosh HD?
7. Click the Erase button in the toolbar, then enter the requested details:

• • Name: Macintosh HD
  • Format: APFS or Mac OS Extended (Journaled), as recommended by Disk Utility

1. Click Erase Volume Group. If you don't see this button, click Erase instead.
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4. If asked, enter your Apple ID. Forgot your Apple ID?
5. After the erase is done, select any other internal volumes in the sidebar, then click the delete volume (–) button in the toolbar to delete that volume.
6. Disregard any internal volume named Macintosh HD or Macintosh HD - Data, as well as any volumes in the External and Disk Images sections of the sidebar. 
7. Quit Disk Utility to return to the utilities window.
8. If you want to start up again from the disk you erased, select Reinstall macOS in the utilities window, then click Continue and follow the onscreen instructions to reinstall macOS.

If you don’t see Macintosh HD in Disk Utility

Your built-in startup disk should be the first item listed in the Disk Utility sidebar. It's named Macintosh HD, unless you changed its name. If you don't see it there, choose Apple menu  > Shut Down, then unplug all nonessential devices from your Mac and try again.

If your disk still doesn't appear in Disk Utility, or Disk Utility reports that the erase process failed, your Mac might need service. If you need help, please contact Apple Support.

Second Part to follow

How to reinstall macOS

Use macOS Recovery to reinstall the Mac operating system.

Start up from macOS Recovery

Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:

Apple silicon

Turn on your Mac and continue to press and hold the power button until you see the startup options window. Click the gear icon labeled Options, then click Continue.

Intel processor

Make sure that your Mac has a connection to the internet. Then turn on your Mac and immediately press and hold Command (⌘)-R until you see an Apple logo or other image.

If you're asked to select a user you know the password for, select the user, click Next, then enter their administrator password. 

Reinstall macOS

Select Reinstall macOS from the utilities window in macOS Recovery, then click Continue and follow the onscreen instructions.

Follow these guidelines during installation:

• If the installer asks to unlock your disk, enter the password you use to log in to your Mac.
• If the installer doesn't see your disk, or it says that it can't install on your computer or volume, you might need to erase your disk first.
• If the installer offers you the choice between installing on Macintosh HD or Macintosh HD - Data, choose Macintosh HD.
• Allow installation to complete without putting your Mac to sleep or closing its lid. Your Mac might restart and show a progress bar several times, and the screen might be empty for minutes at a time.

After installation is complete, your Mac might restart to a setup assistant. If you're selling, trading in, or giving away your Mac, press Command-Q to quit the assistant without completing setup. Then click Shut Down. When the new owner starts up the Mac, they can use their own information to complete setup.

Other macOS installation options

When you install macOS from Recovery, you get the current version of the most recently installed macOS, with some exceptions:

• On an Intel-based Mac: If you use Shift-Option-Command-R during startup, you're offered the macOS that came with your Mac, or the closest version still available. If you use Option-Command-R during startup, in most cases you're offered the latest macOS that is compatible with your Mac. Otherwise you're offered the macOS that came with your Mac, or the closest version still available.
• If the Mac logic board was just replaced, you may be offered only the latest macOS that is compatible with your Mac. If you just erased your entire startup disk, you may be offered only the macOS that came with your Mac, or the closest version still available. 

You can also use these methods to install macOS, if the macOS is compatible with your Mac:

• Use the App Store to download and install the latest macOS.
• Use the App Store or a web browser to download and install an earlier macOS.
• Use a USB flash drive or other secondary volume to create a bootable installer.

Published Date: April 06, 2021

What to do before you sell, give away, or trade in your Mac - Note - This is the proper way as per Apple

Actually - No.

These Forums are only apple user to apple user and for sure no direct access to your computer.

I am sorry to say :-((

The best that can be done is answer any addition questions and / to guidance questions.

When you perform this action - you will require an active and reliable Internet Connection.

As you start the computer and holding those key - it starts to load BUT watch carefully for the Wifi Icon to appear somewhere on the screen. You Must click on that Icon and Manually input the Wifi Password. Only then with " Things Happen " - the Apple Globe will appear and start loading only as fast as your Internet Connect allows.