How to maintain version control of two projects with the same basis?

Question

PHP

Hello.
I have a project that sells paper books.
Everything would be fine, but I am creating a project that will sell audio books.
The very basis of the projects is the same, but there are some functions and features that need to be implemented either on a paper book project or on an audio book project.
At the moment I actually have 2 projects with the same foundation. I don't think it makes sense to shove everything into one project.
And here I need your help. How to properly implement control over two projects so that it would be possible, if possible, to improve, for example, a paper book project, without affecting the audio book project?
There are also global changes or fixes that will affect both projects at once.
Therefore, I can’t just take and divide 2 projects into two directories, as there will be improvements that affect two projects at the same time.
I hope I described clearly.

Answer 1

Everything is simple. 3 separate repositories are made:
1) Core
2) Paper books
3) Audio books
Paper books and Audio books simply use the core that you update and there the changes occur on two sites