Even when you’ve been using Catalina or later for a while, their split startup volumes can confuse. If you’re just making your way up from Mojave or earlier, this is likely to be baffling. Go find something you used to be able to locate easily, like the XProtect data files, or a hidden folder which should be at the root of the Data volume, and you can end up going round in circles. This article tries to explain why things are where they are, so you understand where to look.

When Catalina divided the startup volume into two, System and Data volumes, it did so into a special Startup (or Boot) Volume Group. The two volumes aren’t equals, and don’t appear in the same places in the Finder or in paths in Terminal. Other volumes, such as those on external disks, are normally listed in the Finder’s Locations, and in the top-level Volumes directory, but you won’t find the Data volume there as it’s mounted through the System volume.

When your Mac starts up, it mounts the System volume at its root, and then attaches its paired Data volume quite differently. The Data volume actually appears in a list of other volumes which you won’t use, in Macintosh HD > System > Volumes, where it’s confusingly given the same name as the System volume, by default Macintosh HD. View the contents of that, and you’ll see for example a folder named Applications, which contains all the apps installed on both your System and Data volumes merged into a single list. But look within that at Macintosh HD > System > Volumes > Macintosh HD > System > Library, and the folders you’ll see there don’t include what’s on the System volume.

This is because of the way that the Data volume is attached to the System volume, using special two-way firmlinks. In the case of the Applications folder, the composite list which you’re used to seeing is a combination of those installed on the System volume, which you can see on their own in the top-level Macintosh HD > System > Applications, and those on the Data volume in Macintosh HD – Data > Applications.

This can sometimes become clearer when you show hidden folders (Command-Shift-.) and select Macintosh HD in Locations. That shows what’s at the root level of the System volume alone, from hidden files and folders like .file down to the Volumes folder.

From there, select the System folder, then Volumes, and Macintosh HD:
Macintosh HD > System > Volumes > Macintosh HD displays the top level of your Data volume, starting with hidden folders like .DocumentRevisions-V100 and .fseventd, going down to usr and Volumes.

It’s simple to recognise that as the Data volume, as all of those files and folders can be written, so they must be on the Data rather than System volume. That’s generally a good test of which volume contains any given folder: if its files can be written to, then they must be on the Data volume. If the files are installed by a macOS installer and can’t otherwise be written to, then they’re almost certainly going to be on the System volume.

Apart from the Applications folders, there are some trick folders which you should know about. Starting from Locations:

• Macintosh HD > System > Volumes > Macintosh HD > System > Library contains mutable additions to the System volume, such as CoreServices > CoreTypes.bundle.
• Macintosh HD > Library > Apple contains more mutable additions to the System volume, notably System > Library > CoreServices, where you’ll find MRT and XProtect.
• Macintosh HD > usr > local contains user-installed command tools, in its bin, sbin and other folders.

Unless you regularly use Terminal, trying to remember its equivalent paths for each of these becomes profoundly confusing. If you only use Terminal occasionally and need to access a path on the System or Data volume, locate it first in the Finder, then drag and drop that folder to your command line in Terminal. macOS will automatically generate the correct path to be used there, sparing you the pain of doing so.

For example, the root level of the Data volume, Macintosh HD > System > Volumes > Macintosh HD, translates into a path of /System/Volumes/Data, and the root level of the System volume is of course just /

These layouts and paths have remained largely consistent across Catalina, Big Sur, and should do so in Monterey too. The only slight catch here is that the volume named Macintosh HD – Data by default has increasingly been referred to as just Data, which is also its name on the internal SSD of M1 Macs.

For all the details, this article provides full roadmaps which may help when you get lost.