Create git server with many repositories

Please consider my question from a technical point of view. I am faced with the problem of access control to git 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)))

In GitLab you work with projects and project groups, and assign rights to these. The official documentation explains it well.

You can check gitolite project code. It is very simple command-line-only system to manage multiple git repos with multiple users and different access permissions.

It creates a single ssh user gitolite with multiple ssh public keys put in authorized_keys. It sets "command=gitolite-shell username" in authorized_keys for each individual ssh public key to call its gitolite-shell wrapper for a user. The wrapper then uses gitlite config to determine calling user access rights to each git repo.