My mental model of window sets:

— A window always belongs to one set, with two exceptions: 1) global ephemeral windows like “About This Mac” (they appear in all sets on current desktop) and 2) windows assigned to all desktops via Dock (they appear in all sets and on all desktops).

— You are always “in” a set, even if it’s a 1-set with only this window alone, and you don’t see any windows not in the current set (other than the exceptions above). You can only glimpse other windows via Recent applications, Mission Control, and so on.

Managing window sets:

— Creating a window always drops you into a fresh set with that window alone. While jarring at first, it’s key to me—with a new window I start doing something different and don’t need the distracting baggage of whatever was on screen before that. Can always add windows later (see below).

— Any way that focuses a window now also drops you into its entire window set. This includes: Mission Control (which like before operates individual windows, not sets), ⌘+tab, clicking on app’s Dock icon, window switch from within the app, and so on.

— If you switch to another window set while dragging a window, that window becomes part of the set when you release the drag.

— To remove a window from a multi-window set (creating a new set with only that window alone), long-press the maximize window button and click “Remove Window from Set”.

Recent apps:

— “Recent applications” shows two kinds of things: 1) multi-window sets you created (last focused window on top), and 2) solo window sets grouped by application.

— Recent applications list is scoped to windows on current desktop and excludes the window set you are currently working in.

— You can interact with Recent apps in three ways: 1) click on a set to switch to it, focusing topmost window; 2) click on any app’s icon to show up to 18 recently used window sets with any of that app’s windows; 3) drag the topmost window from any visible set into current set.

— I set it to auto-hide, I only need it when I need to add another window to the current set.

By the way, ⌘+` now works in two ways: 1) if you are in a set of multiple windows, it will switch between windows in the set (regardless of the app); 2) if you are viewing a solo window, it switches between other solo windows of the app (close to old behavior).

This all might sound complex but a couple hours in it works quite intuitively, I find.