Self hostet Gitlab - problem with SSH authentication between servers

Hello there!
I’m new in GitLab, and so do I with self-hosted Gitlab in my company.

Recently, Ive created a new local server for developers. It should connect to local Gitlab instance, they are mainly using this kind of string: git clone git@local_git_IP - and it should allow them to entry without any prompts or passwords. But... Its working on old servers, when it was configured by previous administrator, and he left without any documentation. And here I am, struggling with SSH authentication between two servers.

My case is pretty simple in general:
I want all my devs that have localhost accounts on stage server to connect to my self-hosted Gitlab without passwords, using git@local_git_IP-like commands.
Its well known for me how to connect two “regular” servers - generating keys, authorizing them, exchanging thems. And it works.
But here, with this Gitlab server, Im lost.

Ive created user "git" on stage server. Then Ive generated SSH key and copied the id_rsa string. Then on Git server - there is no /home/git/ directory, but I found out that after: su git > cd > cd .ssh > nano authorized_keys - I can see a bunch of keys, but with some directories/commands in front of them. And adding a new one isnt working, my devs cant clone repositories because of various reasons: connection refused, authenticity cant be established, etc.

Basically, I need a pretty straight help in how to establish a secure connection between a new server and self-hosted Git repository. We can import SSH keys to every dev Git account, but that’s not the case - I just want to know how the heck it was configured and how can I reproduce it.

Sorry for my ignorance or little knowledge - I`m new in Git and I need a solid slap of knowledge, straight into my face.
Thank you SO MUCH!

I’m a little confused — what is the purpose of this server?

Developers can install Git, and SSH client on their desktop directly, and add the SSH public key into their GitLab user profile. GitLab then generates the authorized_keys file you’ve found on the GitLab server.