Internet Explorer was originally based upon the Mosaic browser. Like a lot of Microsoft’s tech, it’s something they acquired.
Up to IE4, it was a standalone browser. It’s IE4 where Microsoft integrated it into the OS and made do double duty as Windows Explorer, which is what you’re thinking of.
I’ve been wanting to play that. Considering it already takes me something like 30-40 hours to launch a rocket in base game, I’m anticipating that getting through the DLC is going to keep me busy a while.