I am learning about Add ins and have found quite a bit in this group.
I could however not find anything explaining the following:

I learned that Add Ins must be "added" and "installed", but sometimes
it might also have to be loaded in order to take effect. I have a few
add ins that are available everytime I open Excel, but these are not
"loaded" (opened) each time - they were installed some time ago. So
does Excel remember the add ins from the previous time it was loaded
or is there an entry made in a register when the add in is
"installed"? Does Excel then refer to the register to see which Add
Ins must be "loaded" when opening the application?

Also, could someone point me in the direction of some useful sites
where I can learn more?