Only one choice in Default Browser drop down

Typically you always have an option to:

> Open With

or

>Other

from the dropdown/popup message.

If you still have issues, holding the Control key and click open

Open a Mac app from an unidentified developer - Apple Support

Safely open apps on your Mac - Apple Support

Chrome must be currently installed on the Mac before it will be presented as an option.

If it is not, you will have to reach out to Google to get it.

Just Run Chrome. I think you'll get a message asking if you want to make it the default browser.