What is the default file system used by macOS since version 10.13 High Sierra?

The default file system used by macOS since version 10.13 High Sierra is APFS (Apple File System). Apple introduced APFS as the successor to HFS+, also known as Mac OS Extended, which had been used since 1998.

APFS was announced at Apple's Worldwide Developers Conference in June 2016 and became the standard file system for macOS with High Sierra, released in September 2017. It was designed especially with modern flash and solid-state storage in mind, while also supporting features such as encryption, snapshots, crash protection, and copy-on-write file cloning.

A common mix-up is assuming every Mac volume is automatically APFS. High Sierra automatically converted compatible internal flash-storage volumes, while hard-disk and Fusion Drive support arrived more fully with macOS Mojave. External drives can still use formats such as HFS+, exFAT, or FAT32 when compatibility with older Macs or Windows is important.