Sorry, can you clarify? What is the "they" in that sentence?

EDIT: a sibling comment jogged my memory; the thing I was doing a bad job of remembering was prebinding. https://opensource.apple.com/source/cctools/cctools-622.5.1/...

When a new version of the OS is installed, the OS re-runs prebinding for applications to the dynamic libraries they rely on because the dynamic libraries may have changed. The alternative is to have a big latency bump every time an app is launched for the first time as the caches are rebuilt on-the-fly.

(I think this is the step that used to be called "Making Your Macintosh Happy" during an install, but the veracity of that notion has fallen too far down the Google search hole for me to easily pull up a reference to confirm or deny it).

That's no longer done on install. The modern equivalent of it is part of the signed image that you download from Apple.