This article is a year old, but I came across it and want to highlight it anyway.
On 27 March 2017, Apple made one of its biggest corporate gambles. When it rolled out iOS 10.3 that day, the installer silently converted the storage in each iPhone and iPad to the first release of Apple’s new file system, APFS. Had a significant percentage of conversions gone wrong, Apple would have had a disaster on its hands, particularly as it didn’t admit to doing this until WWDC just over two months later, when it announced that APFS was coming to macOS 10.13 High Sierra that September.
The conversion of god knows how many iPhones and iPads to APFS, entirely silently, is one of those moments where Apple really flexed its engineering muscle. Since file systems are a bit of an archaic topic these days, I find that Apple really isn’t getting the recognition it deserves for this silent migration to APFS.
AFAIK, this filesystem has fragmentation built-in as a feature, which is why it really really sucked on spinny disks.