where do I find my external device when I connect it to my iMac 24

Sometimes it will appear on the desktop, depending on your settings, but you can always go to Finder > Go (top menu bar) > Computer. It will always appear there, unless there is a problem.

Cheers,

Jack

In Finder's sidebar. You may find this link very helpful:

https://support.apple.com/guide/mac-help/customize-the-finder-toolbars-on-mac-mchlp3011/mac#:~:text=Customize%20the%20sidebar-,On%20your%20Mac%2C%20click%20the%20Finder%20icon%20in%20the%20Dock,choose%20View%20%3E%20Show%20Toolbar.)