It’s not something that everyone’s expected to know when starting out, don’t worry about it.

Yes, if a shop is disused, the POI remains, typically with:
 - shop=vacant
 - old_name=Shop’s Old Name (since it might still be on signage or locally referred to as that)
 - addr:* if the address is known

There’s some more information about the tagging here: https://wiki.openstreetmap.org/wiki/Tag:shop%3Dvacant. Generally searching the wiki is a good way to go about finding about different tagging schemes.