All Contacs vs All iCloud - how to merge?

In my Contacts app (v13.0) on Big Sur, in the left hand Groups column, I now have All Contacts, iCloud, On My Mac, and one of my Gmails. They seem to operate as separate lists. In some of the cards, there's actually a "Cards" section and it may have both iCloud and On My Mac, and even a 3rd (Gmail) as cards. Yet some cards have ... no cards section. It's totally bizarre.

I've got no need for having separate cards in separate groups, not sure how they ever got into separate groups.

I've tried highlight All Cards, and dragging them into All iCloud, and that actually works, but it leaves me with hundreds of duplicates. Which is another thing: Contacts' "Find Duplicate" option definitely does not find all of the duplicates.

I'm quite confused. Is there a PDF or doc or something where that explains how some of these cards of my contacts (I've got around 4,000) have gotten into these separate groups, and how to eliminate those groups and just have everything in All iCloud (and therefore obviously All Contacts). And how to remove the "card" section in some of the vCards.