That’s how it originally worked, but Apple has moved away from PDF to other native GPU frameworks for drawing. Now window content is stored as bitmaps instead of redrawing vector instructions.

That's not how it ever worked.

Quartz is a vector drawing engine whose output can be captured as a PDF and which is closely aligned with the PDF imaging model.

Window content was always stored as bitmaps.

You’re correct. I meant to say textures, but even that isn’t completely accurate.

AppKit used to generate the bitmap directly through CoreGraphics (PDF model). Now it uses Core Animation and Metal and no longer draws the bitmap directly.

You have understood things incorrectly.