Using composer on a specific folder via SSH in centOS 7 with WHM/cPanel

Question

Composer seems to be the way to go to install PHP dependencies.

I don't know if it's used only for PHP or not, but:

I want to install PHPMailer for a specific domain in a VPS managed with WHM and cPanel.

I am in the console via SSH as root. So I guess the "current folder" is the super root folder of the server. What for windows would be C:.

How may I navigate to a specific folder inside a domain that was installed as a cPanel account?

Let's say I am in a vps.example.tld and there is a whatevertomain.tld as an account of that VPS.

How may I run composer install phpmailer in the folder whateverdomain.tld/libraries/phpmailer so that I can run phpmailer from that said folder?

Maybe I am mixing up too many terms, but any help will be very appreciated.

Thank you.