Spotlight index for external drive

I have a large external drive with documents. When I plug the drive into the system (Catalina) will spotlight index the external drive automatically and keep this index for the second time I connect the drive ?

How can I best index the external drive and maintain the index on mount and I unmount ?

it should do that by default, I think. [update hour after this reply: I checked, and it definitely should]
If not you can check if the drive is excluded under the “Privacy” tab at the top of the Spotlight preference pane.