Change all linked objects? I have tried to understand linking objects (or is it meshes?), but am apparently failing. I want to create a 2nd and 3rd copy of a collection such that if I change any of the objects, they all are changed. It's fine if one has to be a master copy that has to be the one changed. I have tried various approaches including selecting the objects in the collection then use Duplicate Linked, and it always makes copies that are not updated when one is changed. I have been using links to copy modifiers to other objects, but again it copies the modifiers but doesn't link them together in any way I can see. I have only been working with simple cubes.
Can someone recommend a better source for how to do this than I've found so far (or point out what I am misunderstanding)?
Duplicate linked (alt d, or &quot;duplicate objects&quot; operation with &quot;linked&quot; enabled in operator panel)  should work.  I can't say why it's not working for you, not without a file and procedures to follow that don't do what you expect.
However, there is something else that you can do, which is to simply change the geometry that an object refers to.  If you select an object and look at properties/object data, the very first thing on the tab is a drop-down that specifies which geometry is used for the object.  You can assign any (mesh) object any geometry datablock that exists in your scene.  At that point, the geometry is linked:

There's no master object for linked meshes; editing any of them should edit all of them.  Note that some things, like shapekey values, are still per-object, so you can edit a shapekey for the linked mesh data and then choose to use it or not on a per-object basis-- perhaps this is what was giving you trouble?  Editing a shapekey?
Linking modifiers isn't the same thing as linking mesh data.  It's basically a &quot;copy modifiers&quot; operation, and future changes made to one won't affect the others.