Create git server with many repositories

I'm going to create a product similar to GitLab, or GitHub. Please consider my question from a technical point of view.

I am faced with the problem of access control to repositories. I need to somehow distinguish the access of certain ssh-keys. That is, the key should give access to one repository, not to all.

I looked at how git servers are organized: a git user is created (that's what it's called, I'm shocked))), a repository (one) is created and shh keys of development participants are added. So, I need this repository to be not one but many, but the key did not give access to all depositories.

If you know how such services and products as GitLab, Gitlab, gitlab are organized, I really want to know how to organize access. I will be glad of any comments, questions, answers, thank you in advance for your attention to the question)))

At scale, you're going to need a database or database table of some sort associating a specific public key with a specific user account (or set of accounts). This solves several problems:

1. Users can manage their own keys
2. Users can only interact with repos where they have permission
3. Users can create service accounts or deploy keys for automated systems (especially CI/CD)
4. Changes are immediately available to every worker that connects to that database (otherwise you'll have to wait for something to update each worker in turn)
5. File permissions for files on block storage becomes much easier (the git user owns everything, and access is managed through your code's permissions model)

Of course, now you have to manage a database, complete with replication and backups and failover plans - but if you want to replicate all the extra GitHub/GitLab/etc. bells and whistles on top of vanilla Git, then you'll have one of those anyway.