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Apple makes it incredibly difficult to manage usage of the disk, for instance, they only offer you two choices for Photos: All photos locally, or "Optimize disk storage" - i.e. Apple Magic.
iMovie too uses a "Library" which is code for "Apple Magic." If you have a project in progress and want to move it to a USB disk because you need the space on your tiny SSD to download a single game, good luck actually getting that simple task done, because everything including your imported clips and your project files, is in a "Library."
Basic users have no idea how to handle these "libraries," and honestly, neither do I, beyond being able to forcibly move the entire library "bundle" to an external drive, which may as well mean deleting it, since this stuff expects to be left alone in-place.