Here are just a few of the methods Apple is known or believed to use to catch leakers…

Origin: 9 to 5 mac – How Apple catches leakers: From color changes to comma placement

Invisible watermarks
We’re all familiar with visible watermarks used by sites to ensure that original images are properly credited, but it’s also possible to embed watermarks which are invisible to the naked eye, but can be digitally detected.

For example, this square appears to be all black: In fact, one section of it is #0D0D0D instead of #000000.

Filenames
Another reason never to use original images is that it’s very easy for Apple to use unique filenames, for example:
very_secret_image_46793459583203.jpg
very_secret_image_46793469583203.jpg

Serial numbers
Document serial numbers is another variation. For example, when Apple shares videos with employees, each is watermarked with an ID number which is likely cross-referenced with the Apple Connect ID of the member of staff.

Subtle typeface changes
Many images contain text, and subtle typeface changes are one easy way to create unique versions. For example, with a serif font, a version could be created with a single pixel missing from a single stroke of a single instance of a single letter. Font sizes can also be scaled up or down by as little as one pixel.

Non-subtle typeface changes
Sometimes, Apple uses the opposite approach, and chooses very unsubtle changes. If it wants employees to be very aware that their copy is unique, it has used things like random italics or bold. For example:
This year, the iPhone 15 will launch on Tuesday August 29 instead of the usual September timing.

Even though an employee may retype something, rather than copying and pasting, it will make them fear other identifying features, such as…

Wording or punctuation changes

Tiny changes in wording are trickier if you’re trying to provide a unique copy to a lot of people, but can be very helpful once you’ve narrowed it down to a handful of people. For example:

This year, the iPhone 15 will launch on Tuesday August 29 instead of the usual September timing.

versus:

This year, the iPhone 15 will be launched on Tuesday August 29 instead of the usual September timing.

versus:

This year, the iPhone 15 will launch on Tuesday August 29, instead of the usual September timing.

The addition or removal of something as subtle as a comma can be enough.

False information
A simple approach with things like specifications is to have a tiny change of detail in each copy of the document. This would, obviously, need to be to an element the employee concerned isn’t working on.
For example, for an employee whose work does not relate to radio bands, imagine how easy it would be to change a single digit in this:

Or for one working on radio bands, changing f/1.78 to f/1.76 in this:

Fake dates, prices, colors, and more are possible…