How to have contractors share code?

tldr: How do I get the code that I've commissioned?

I've farmed out various projects over the years, but I still don't have a good idea as to where my original code lives, if anywhere.

I know I've lost some on Dropbox, I have some spread out in Bitbucket and Github, some in gmail, some in gdrive, some lots on old hard drives, some on drives or githubs of the various developers/agencies i've hired, etc.

I've tried various things in the past, like adding agencies to teams on my githubs, etc. -- it all seems insane from a maintainability/security point of view.

What is the easiest, lowest-maintenance way to get access to the code I am having developed?

Thanks.