Problem with Terminal Utility

... I've never used Terminal before ...

Then just run the command suggested, as if you are not a hard core Terminal user, the suggested change will not really make any difference to you, and it will stop that message from coming back:

chsh -s /bin/zsh

Quit the Terminal, and the next time you launch Terminal (if you ever have a need) that message about changing your shell will not appear.

NOTE: If you really want to know more about shells, their different flavors, what they do for you, why they are called a shell, etc... we can address that as well, but based on what you have said so far, working in the Terminal is not your normal thing, and all this Unix take is not for you.

It means that the default command shell—the syntax of the commands entered at the command line, and command-line scripting—is now zsh, and not bash.

This is described in Use zsh as the default shell on your Mac - Apple Support

Basically, the bash command shell you’re using is getting stale, and Apple is shifting folks over to the zsh command shell.

If you do not want to see that diagnostic, you can follow the directions in that article:

If you invoke the bash shell while macOS Catalina is configured to use a different shell, you'll see a message that the default interactive shell is now zsh. To silence this warning, you can add this command to ~/.bash_profile or ~/.profile:

export BASH_SILENCE_DEPRECATION_WARNING=1

Or ignore it. Or use the command shown, and shift your own login user default command shell from bash to zsh.

As for your goal: here is How to create a bootable installer for macOS - Apple Support using a flash drive. I wouldn’t bother with an external hard disk, as those are usually far larger than a flash drive, both physically and in terms of the necessary storage capacity. But it’ll work. And it’ll work from either bash or zsh.

Run the installation media creation command in the how-to-create-an-installer article.

There’s seemingly little reason to be distracted by the zsh shell-transition diagnostic. Which is benign.

One common cause of macOS instability can be add-on security tools, add-on cleaners, add-on VPN clients, and related. That’s not specific to Catalina.