To what extent does it matter where my code is? That is, whether the
code sits behind a worksheet, or in a module, or behind a userform?

I assume that if I have controls on different objects with the same
name, the code behind that object will control it. For example, if I
have a "Show Instructions" button on a worksheet and a userform, both
with the name ShowInstructions, then the ShowInstructions_Click() event
behind the worksheet and the form will each respond to the button on
the worksheet or form.

But what about code in a module? Does VBA care if its in a module or a
worksheet? Does it share values in a given object but not between
objects? What exactly do the "public" and "private" statements do in
regard to variables, and to objects and events? In the programming I
have done, I've often been casual or careless about this stuff, and
have usually been able to get it to work, but I'd like to understand
better and make my programs better and cleaner, as well as more
functional.

Thanks in advance.