How can I download and install Mojave on an external drive?

Download the installer normally, and then either specify the external drive when setting up the installation or create an install thumbdrive.

(190978)

You can download from this:

https://support.apple.com/en-gb/HT211683

Use Safari to open the link, other web browsers may not help.

https://itunes.apple.com/us/app/macos-mojave/id1398502828?ls=1&mt=12