How to maintain a large project in a version control system?

Question

git

Let's say there is a large project that includes many applications and libraries. Also, this project can be customized for many customers (which will entail changing some applications and libraries). Interested in answers to the following questions:
1) How is it easier to store such a project (monolithically or separately)?
2) How, in this case, to take into account the possibility of customizing the entire project?
3) How do you save releases?
4) How to apply critical patches?
5) How does it work in large companies? (Google, Microsoft)
PS I'm aware of how the Linux kernel is maintained, but I'd be happy if you provide other similar open-source examples.