@theluthier about linking and Collections – linking does appear to be how you put a single object into multiple collections. Instead of using M to move it to another collection, Shift-M links an object to another collection. The link isn’t a duplicate object, it just serves as a placeholder for that object in that collection, that you can use the collection features on (turning visibility or selectability on and off for example). Think of it like a shortcut in your operating system. You can also link an object into another collection by dragging it in the outliner to the collection you want to link it in, and holding Ctrl before dropping it.
Selecting a linked object in a collection is the same as selecting the “original” object, there’s no distinction. So there is no special need to keep track of what collection the “real” object is in versus which collections it’s only “linked” in. They are all the same object.
So let’s say you have a Cube in Collection 1, and you want it to also be in a Collection 2. So you hit Shift-M > New Collection, name it Collection 2 and hit OK, and now in your Outliner, Cube is in both Collection 1 and Collection 2. Now let’s say some time later on you decide you don’t want the Cube to still be in Collection 1 after all. In the Outliner just go to Collection 1, right-click the Cube, and choose “unlink”, and bam – now it only exists in Collection 2.
