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.
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.