Thanks for the link. Good article, but I wished it went a little further.
For example, if code from a previous AV is still being loaded even though it’s been installed,
where is it loaded from? Surely there has to be a reference in a certain part of the registry
for it to be loaded.

Are left overs to be found in the device manager, perhaps non-plug and play drivers?

If the file is not loaded, but just left in \system32 (for example) it should be harmless.

This area has been danced around, and not really explained for too long.
It would be nice if somebody, who knew the details, could actually provide
a technical explanation.