How to create a user with FTP and SSH access in Debian?

Question

Debian

There are vds on Debian OS without ISPManager etc. That is, you have to do everything manually. There are root rights and one user who has access to the site (with FTP and SSH access).
Questions:
How to create under domains (LAMP stack);
How to create a user and connect it to a specific subdomain (with FTP and SSH access).
Can you explain clearly and in detail?
PS I did not create the first user. Can't ask him/them!