download and SD

What specific nuvi do you have?  Having said that, what you can download as a “lifetime” update, won’t generally include other geographical locations.  For example my nuvi 2689 is entitled to “lifetime” updates to the maps associated to North America.  Because I may have a trip planned to Europe, that won’t entitle me to a map of Western Europe without paying for it.

Nuvi Drive 52 lm

No, you can't. Only your original map can be installed to your PC. What you can do though, if you purchase a downloadable map, is copy the map file to either a physical or virtual removeable drive, eg a memory stick, and that can then be accessed by your PC without the need to plug in your device. I do that with various maps so BaseCamp can use them.

So, if I understand correctly, if I download a downloadable map from garmin.com onto my Garmin device, I can then copy that map onto a memory stick. Or is it the other way around, I download from garmin.com to my memory stick and then transfer to my Nuvi. Which is correct?

Download to your device, copy the downloaded .img file to a memory stick or any other type of removeable drive