How do I find Disk Utility on my old I Mac with Big Sur. I want to reformat the disk

Follow the steps at > Use Disk Utility to erase an Intel-based Mac - Apple Support

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.

4) Select Macintosh HD in the sidebar of Disk Utility. Don't see Macintosh HD?

5) 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

6) Click Erase Volume Group. If this button isn't shown, click Erase instead.

7) If asked, enter your Apple ID. Forgot your Apple ID?

8) Optional: If you previously used Disk Utility to add internal volumes other than Macintosh HD, you can erase them individually using the same process. 

9) When done, quit Disk Utility to return to the utilities window.

10) If you want your Mac to start up again from the volume you just you erased, select Reinstall macOS in the utilities window, then click Continue and follow the onscreen instructions. Learn more about reinstalling macOS.

You cannot erase the internal HD if you are already booted on it. Please follow the steps in What to do before you sell, give away, or trade in your Mac. To erase the internal HD you will need to reboot in the Recovery Partition by holding down Command + R on startup and open Disk Utility there. That is where you can erase the internal drive.

Plug the keyboard in and hit the power button, immediately hold down Command +R keys and hold them down until it boots into the Recovery Partition.

Look in: Applications - Utilities - Disk Utility. OR the most simple method is to hold down Command + Spacebar and type Disk Utility.

That is why you need to startup in macOS Recovery and then use that Disk Utility.

Command R