Three things confuse most people about iPhone storage: the mystery System Data figure, the difference between offloading and deleting, and why an expensive drive copies slowly on some iPhones.
What System Data is, and why it changes
System Data (called Other in older versions of iOS) holds caches, logs, downloaded voices, streaming buffers and temporary files. iOS clears much of it by itself when space runs low, which is why the figure jumps around from day to day. Clearing Safari’s history and website data trims some of it. Only a full backup and restore resets it, and that is rarely worth an afternoon.
Offloading vs deleting an app
Offloading removes the app itself but keeps its documents and data, and leaves its icon on the Home Screen with a small cloud beside the name. Tap it and the app reinstalls from the App Store, provided it is still listed there. Deleting removes the app and its data, unless the app keeps its own copy in iCloud. Offload big apps you use now and then; delete the ones you have stopped using.
Why USB-C speed depends on which iPhone you own
Every iPhone from the 15 onward has USB-C, but only the Pro models run it at USB 3, up to 10 Gbps. The iPhone 15, 16, 16e, 17 and Air run at USB 2, 480 Mbps, which is roughly 40 MB/s in real copies, the same class as Lightning. A fast SSD still works on them, just no faster than a good pen drive. Pro owners also need a USB 3 cable, because the one in the box is USB 2.
Drive formats and the Files app
The Files app reads and writes drives formatted as exFAT, FAT32, APFS and HFS+. Format a new drive as exFAT on a computer if it will move between iPhone, Windows and Mac. Drives formatted as NTFS on a Windows PC open read-only, so you can copy files off them but not onto them. If a drive draws more power than the iPhone can supply, iOS shows an error; an adapter with a charging port or a powered hub fixes that.