ssh permission denied when prefixing with ssh protocol - ssh

Question

I try to add a ssh key to my GitLab account in order to use it through npm.
I added my key following https://docs.gitlab.com/ee/ssh/.
ssh -T git#gitlab.com
Is working fine and deliver the message Welcome to Gitlab, username.
But ssh -T ssh://git#gitlab.com is not working and give me this error message:
Permission denied (publickey).
And when I run npm install on my node project where I added my node module as a dependency inside the package.json file, it's returning the following error:
npm ERR! Error while executing:
npm ERR! /usr/bin/git ls-remote -h -t ssh://git#gitlab.com/myproject/myproject.git
npm ERR!
npm ERR! Permission denied (publickey).
npm ERR! fatal: Could not read from remote repository.
npm ERR!
npm ERR! Please make sure you have the correct access rights
npm ERR! and the repository exists.
npm ERR!
npm ERR! exited with error code: 128
I added my dependency inside package.json as follow:
"my-project": "git#gitlab.com:myproject/myproject.git",
I'm not comfortable with ssh and key pairing, if anyone could tell me what is wrong with my configuration it will help me a lot, thanks.

Answer 1

It's kind of funny to be misled by the following line (talking about myself):
ssh -T ssh://git#gitlab.com
The command doesn't expect actually a protocol prefix, but a hostname prefixed or not by a user name followed by an arobase, as said the manual page:
ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port] [-E log_file] [-e escape_char] [-F configfile] [-I pkcs11]
[-i identity_file] [-J [user#]host[:port]] [-L address] [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port] [-Q query_option] [-R address]
[-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]] [user#]hostname [command]
Here, it takes actually ssh://git as a user name. That's why it won't log in since the user name which whom you connect has to be git, and not ssh://git.

Answer 2

What's the difference between "SSH", "ssh" & "ssh://"?
SSH is a transport layer protocol. link
ssh is an OpenSSH SSH client. OpenSSH is the connectivity tool for remote login with the SSH protocol. link
ssh:// is a Uniform Resource Locator (URL). link
ssh -T git#gitlab.com - Using an ssh client, log in to gitlab.com with username as git
ssh -T ssh://git#gitlab.com - Using an ssh client, log in to gitlab.com with username as ssh://git (Thanks Amessihel for the detailed answer)
ssh://git#gitlab.com/myproject/myproject.git - Instructs the application accessing this URL that it requires ssh client to access, and if there an ssh client installed on the operating system, opens the SSH client, log in to gitlab.com with username git and access the myproject/myproject.git repo. Chrome understand this URL, copy this URL on the chrome address bar and hit enter.
Just like any URL, for example, https://google.com. Many clients support HTTP, like wget, curl or a web browser, and for HTTP & HTTPS URL operating systems will open web browsers.