Where Are iCloud Drive Files Stored Locally on Mac?
The local iCloud Drive cache folder on Mac, and what Optimize Mac Storage changes about it.
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Finder's iCloud Drive is a view onto ~/Library/Mobile Documents/com~apple~CloudDocs (quote the space in Terminal). Sibling app containers like Pages sit under the same Mobile Documents root.
Optimize Mac Storage decides how much of it is really on disk. Off means full local copies, which is what you want for complete Time Machine backups. On means recent files stay while older ones become on demand placeholders, and backup tools cannot back up a placeholder. The bird daemon runs sync with state in ~/Library/Caches/com.apple.bird; diagnose with brctl rather than deleting that cache, which triggers full resyncs.
Where macOS stores this, by platform
~/Library/Mobile Documents/com~apple~CloudDocs/
What Finder shows as iCloud Drive. Escape the space in Terminal (Mobile\ Documents). Optimize off keeps full files (needed for real Time Machine coverage); Optimize on evicts old files to placeholders. Sync state in ~/Library/Caches/com.apple.bird; use brctl log/download/evict. Some releases also show a CloudStorage alias; this path stays canonical.
Frequently asked questions
Why doesn't Time Machine back up my iCloud Drive files?
Because Optimize Mac Storage turned them into on demand placeholders, which contain no data to back up. Turn Optimize off (or mark folders available offline) and let files download fully before the next backup runs.
How do I force an iCloud file to download or evict from Terminal?
Use brctl download <path> and brctl evict <path> on the file inside com~apple~CloudDocs. Finder right click offers the same download and remove download options without commands.
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