IPSW files have always been ZIP files, but the exact details of the format have changed over time, especially with respect to the root filesystem.
| iOS version | Rootfs DMG compressed in the ZIP | DMG compression | Root filesystem | FS compression (decmpfs) |
|---|---|---|---|---|
| 10.0.1-10.2.1 | deflate | UDZO | HFS+ | zlib (3/4) |
| 10.3-11.? | deflate | UDZO | APFS | zlib (3/4) |
| 11.?-12.5.7 | stored | UDZO | APFS | zlib (3/4) |
| 13.0b2-13.7 | stored | ULFO | APFS | zlib (3/4) |
| 14.0-15.7.2 | stored | ULFO | APFS | zlib (4) and raw (9) |
| 16.x | stored (but cryptex deflated) | ULFO | APFS | zlib (4) and raw (9) |