Earlier quoted context omitted.
No, that is completely wrong and would be nuts. The only way the whole session gets elevated is if you'd launch explorer.exe with an admin token. The way privilege escalation works on Windows is that pretty much everything gets launched with a standard user access token by default, and processes can request an admin access token in a few ways, UAC being the main one. When a process is supplied that token, that proces…
(Only) the process is elevated, but the process has a window on a shared session, and the OS does not successfully protect processes that share a session (and user, and registry, and disk, etc., etc.) from controlling each other. From an API point of view, only one process is elevated. From a security point of view, if one process is elevated they all are, due to a lack of any effective mechanism that actually stops…
I do fully agree that desktop OSes are a legacy security model and they can't hold a candle to that of iOS. Android is getting there, but because it also started from mostly an open all-access model it's been having the same warts.