This adage translated to classic MacOS becomes "Everything is a resource". The Resource Manager started out as developer cope from Bruce Horn for not having access to SmallTalk anymore[0], but turned out to completely overtake the entire Macintosh Toolbox API. Packaging everything as type-coded data with standard-ish formats meant cross-cutting concerns like localization or demand paging were brokered through the Resource Manager.

All of this sounds passe today because you can just use directories and files, and have the shell present the whole application as a single object. In fact, this is what all the ex-Apple staff who moved to NeXT wound up doing, which is why OSX has directories that end in .app with a bunch of separate files instead. The reason why they couldn't do this in 1984 is very simple: the Macintosh File System (MFS) that Apple shipped had only partial folder support.

To be clear, MFS did actually have folders[1], but only one directory[2] for the entire volume. What files went in which folders was stored in a separate special file that only the Finder read. There was no Toolbox support for reading folder contents, just the master directory, so applications couldn't actually put files in folders. Not even using the Toolbox file pickers.

And this meant the "sane approach" NeXT and OSX took was actually impossible in the system they were developing. Resources needed to live somewhere, so they added a second bytestream to every file and used it to store something morally equivalent to another directory that only holds resources. The Resource Manager treats an MFS disk as a single pile of files that each holds a single pile of resources.

[0] https://www.folklore.org/The_Grand_Unified_Model.html?sort=d...

[1] As in, a filesystem object that can own other filesystem objects.

[2] As in, a list of filesystem objects. Though in MFS's case it's more like an inode table...