One root, branches everywhere
Unix filesystems are a single tree rooted at /. There are no C: and D: drives — every disk, every USB stick, every network share gets mounted into that one tree. When you list /, you see the whole world.
The standard top-level directories
The Filesystem Hierarchy Standard (FHS) names each branch:
/— root, the only one/bin,/sbin— essential binaries (boot-time tools)/usr/bin,/usr/local/bin— third-party binaries/etc— system-wide config files/var— variable data (logs, caches, mail)/tmp— temporary files, often wiped at reboot/home(Linux) //Users(macOS) — user homes/opt— optional / third-party packages/dev— device files (every disk, every tty)/proc,/sys— Linux-only kernel views
macOS oddities
macOS adds its own folders: /Applications, /Library, /System, /Volumes. On Apple Silicon, Homebrew lives at /opt/homebrew; on Intel Macs it was /usr/local. Since macOS Catalina the system volume is read-only — you'll see a separate /System/Volumes/Data for writable user data, with hard-link magic so paths still look normal.
Mounted volumes
External disks appear under /Volumes on macOS, /media or /mnt on Linux. mount with no args lists every mounted filesystem. df -h tells you how much each has free.