How do I delete user name and password from startup drive?

On the external drive delete this file then restart from the external drive...

 /var/db/.AppleSetupDone

Once you've done that & you've restarted from the external, the computer starts and it's like the first time you used the machine, except all your data will still be there. Your old accounts are all safe. From there you just change all other account passwords in the account preferences!!

If you're booted from the Internal drive, can't you just use the Finder?

Oh, but if only the external is bootable then yes...

See if you can create a new administrator account by restarting the Setup Assistant:

1. Boot into Single User Mode: Start/restart your Mac. As soon as you hear the startup tone, press and hold ⌘ + S until you see a black screen with white lettering. (If you end up back on the login screen after a flash of the black screen with white lettering, enter your password and it will return to the black screen.)
2. Check and repair the drive by typing /sbin/fsck -fy then ↩ enter - as directed by the on-screen text.
3. Mount the drive as read-write by typing /sbin/mount -uw / then ↩ enter.
4. Remove the Apple Setup Done file by typing rm /var/db/.AppleSetupDone then ↩ enter.
5. Reboot by typing reboot then ↩ enter.
6. Complete the setup process, creating a new admin account.

Be very careful to notice the spaces in those Terminal Commands.

Once you've done that the computer reboots and it's like the first time you used the machine, except all your data will still be there. Your old accounts are all safe. From there you just change all other account passwords in the account preferences!!

It's a hidden file so in finder, Press ⇧⌘. —the command-shift-period chord—to toggle the display of the hidden files, and the Library folder will then become visible in your home directory. And will become invisible, when you press that chord again.

In Finder's Menu, select Go menu>Go to Folder, and go to "/volumes". (no quotes)

Volumes is where an alias to your hard drive ("/" at boot) is placed at startup, and where all the "mount points" for auxiliary drives are created for you to access them. This folder is normally hidden from view.

Mine..

Macintosh HD

OWC Envoy Pro mini

SSD2

Update

Sonoma - Data

SonomaApplications

Macintosh HD - Data

BigSurApplications

SSD3aApplications

Add the external Drives name, SSD2 in my case...

If in terminal enter...

cd /volumes/SSD2/var/db

ls -a

Or...

rm /volumes/SSD2/var/db.AppleDiagnosticsSetupDone

If there's a space in the Volumes name then enclose the name in Quotes...

rm /volumes/"SSD 2"/var/db.AppleDiagnosticsSetupDone

Find Any File...

http://apps.tempel.org/FindAnyFile/index.php

Hold Option or alt key when selecting Find to Find All.

Be certain to set it to search the external drive.