Finder builds a thumbnail for everything, and keeps them in one cache

Every time Finder shows a preview, a Space bar QuickLook, or a Cover Flow style thumbnail, macOS generates an image and caches it. The cache is bounded in principle and grows steadily in practice, particularly on a Mac used for photography or video.

Where it lives

du -sh $(getconf DARWIN_USER_CACHE_DIR)/com.apple.QuickLook.thumbnailcache 2>/dev/null
du -sh ~/Library/Caches/com.apple.QuickLook* 2>/dev/null

The first path is the one that matters and the reason people cannot find this folder: it is inside the per user temporary area with a randomly named path, not in your Library. getconf DARWIN_USER_CACHE_DIR prints it, which is the only reliable way to get there.

Resetting it properly

qlmanage -r cache

That is the supported reset. It clears the store and lets macOS rebuild as you browse. Deleting the folder by hand while Finder is running is the version that produces missing previews until the next restart.

When it is worth doing

• Previews are wrong or blank. A stale thumbnail cache is the usual cause, and resetting it is the standard fix.
• The folder is genuinely large. On a Mac that has browsed tens of thousands of images or video files it can reach gigabytes.
• After deleting a lot of media. The cache does not shrink when the originals go.

What it is not is a fix for a full disk. It is a cache in the ordinary sense: the same trade every cache offers, which is space now against slower previews later.

The neighbours in the same folder

That per user cache directory holds more than thumbnails, and it is part of the temporary area macOS never mentions. A restart clears much of it safely, which is worth trying before any command.