How do I create an album for an externally created directory in a Collection?

I installed digiKam-8.4.0 (in Windows 10), created a locall collection named “Pictures” for P:. This created an album for each directory child of P:.

so far so good.

next, i moved a bunch of directories into P: using a filemanager, ie. outside of digikam. how do i tell digikam to “adopt” these new directories just like it did with the initial set, without copying the files i already have?

In the collection settings, you can tell it to watch for file changes.
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Alternatively, press F5 to force a rescan.