Using an external HD as my applications folder
Yes, you can use an external drive to keep (install) apps.

You should be fine. You don't need to specifically create a folder named Applications on the external drive. Any name or directory hierarchy should be fine.

macOS apps are generally made available as bundles (.app extension) which is technically a directory appearing as a single file to the user (with a specialized structure). An app bundle could be placed anywhere on secondary storage and the app can be used without any difference.

By default, apps are installed in boot volume's /Applications directory and are automatically made available to all the users on macOS. Keeping an app bundle in a folder/drive only accessible to you limits the app available to you.

On a related note, it is worth mentioning for the macOS newcomers that installing/uninstalling an app in macOS is as simple affair as copying/trashing the app's .app file.