PCD is used for migration upgrade and fresh install. It can also be used as an SFTP for backups.
while deploying the PCD, you configure two account one is application and another is os admin account.Application credentials are for web and os admin for cli.
share the application username and password to the vendor and they can do all the admin jobs from the web.
to upload the files to the SFTP folder inside the PCD, use adminsftp as the username and password use the application password.
Kindly read the PCD admins guide to learn more.