Why can't I access the remaining 220GB storage on my 1 TB iPhone 13 Pro Max?

I'm trying to transfer a movie to my iPhone but it gives an error message saying it can't because there isn't enough free space. Settings / General / iPhone Storage shows 787.7 GB of 1.02 TB used (mostly by Applications and Photos). It's annoying not to be able to use the full 1 TB I paid for, any ideas how to access this elusive last bit of storage?

I'll note that I have the 200 GB iCloud family plan (87 of 200 GB used), which seems coincidentally close to the free space I can't access on the phone. But could just be a coincidence.

[Re-Titled by Moderator]