This is a very interesting scenario. There are two main approaches: monorepo and multiple repos. Let's explore the pros and cons of each.
Choosing a source control strategy
Monorepo
In the monorepo approach, your entire code base is in a single source control repository. It is very easy to perform operations over the entire code base. Whenever you make a change, it is reflected immediately in your entire code base. Versioning is pretty much off the table. That's great for keeping all your code in sync. But, if you do need to upgrade some parts of your systems incrementally, you'll need to come up with workarounds, such as creating a separate copy with your new changes. Also, the fact that your source code...