This is what we've built RemNote for! You can add your sources (PDFs, websites, YouTube, etc.) and create cards directly from them. Either with AI (click on a sentence => get an AI card), or by highlighting a sentence and writing your own card.

It's not at the OS level, but I think a focused standalone all-in-one experience is actually better. We've explored a chrome extension here that does something similar to what you suggest, but it somehow hasn't proven useful in practice. I always keep coming back to just doing everything directly in the tool (upload PDF, take notes, make cards), as it helps me focus deeply on what I'm learning.