external drive shows disk activity for several minutes after time machine completes backup; what's going on?

After Time Machine finishes backing up, Spotlight catalogs the drives since it senses a change.

You cannot disable Spotlight from cataloging the Time Machine drives because Time Machine requires Spotlight when recovering files.

I am on Catalina but see this all the time and it lasts longer with rotational drives being used for Time Machine versus SSDs (I use both). My belief is that this is the computer indexing the backup drive since some new or changed files have been placed on it. This is to enable you to search through Time Machine backups. And such activity would be faster on an SSD than rotational drives. If you can, check activity monitor and if you are fast enough you may see some processes named something-"mds" which are indexing activities.

I see the same thing but no issue ever happens.