If you want to know why the APFS file system gets you storage back there are a few reasons. Better partitioning scheme and other low level optimization. Also APFS uses file system features like cloning, which basically means the same data isn't saved twice. If you made a hundred copies of the same file it doesn't actually have a hundred copies, just one with a hundred user visible files pointing at one file. This is kind of a double edge sword, while it preserves space it makes it harder to clear space manually when you need it.
Plus the update cleaned up some of the junk and cache.