APFS or OS Extended fo external HD

It sounds like you’ve made a Bootable USB Installer. A bootable usb installer is not the OS. It is an installer drive you boot from to install macOS on another disk. You would create the installer on some other USB flash memory device which must be formatted Mac OS Extended. Then, you startup your Mac with that device. Then, you install macOS onto your external drive.

All instructions are here: Create a bootable installer for macOS - Apple Support. See the section: Use the bootable installer.

You cannot install onto the drive you used to make th installer.

I prefer to format the drive using Disk Utility.

Anyway your problem is possibly caused by two things.

Make sure you format the Physical Disk and not a Volume.

There is also the business of using the correct socket (DFU or not?) when installing the OS but somebody else can advise on that.

See this Apple Info sheet . . .

How to use an external storage device as a Mac startup disk – Apple Support (UK)

Create a bootable installer for macOS

Connect and rename a USB flash drive

1. Connect a USB flash drive or other secondary volume directly to your Mac. When you create the bootable installer, this flash drive will automatically be erased using the appropriate format: Mac OS Extended (Journaled).

Use this Terminal Command

sudo /Applications/Install\ macOS\ Tahoe.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

When you say it " MacOS Extended once created the bootable disc it cannot be opened "

Why are you attempting to Open the Bootable Installer ?

Normally, when using the correct Terminal Command

The common line will end by saying something like " the Bootable Installer was successfully created "

Self correction and should read " The command line will end by saying something like " the Bootable Installer was successfully created "