MC doesn't automatically transfer the Library (the database of where the files are on disk) but it certainly did not delete any of your files off of your hard drive.
So, they wouldn't have shown up in MC itself when you upgraded. For that, you'd want to restore a backup of your Library from MC19, as explained in
the other thread. But it didn't delete the files themselves.
I'm also confused about what you mean by "install routine". MC doesn't have an installer on the Mac. You just drag it from the DMG and put it in your Applications folder, like most apps. I think, perhaps, you're confused.
If you explain in more detail what exactly happened, calmly and without jumping to conclusions, perhaps someone here can help.