Does .env need to have all fields filled in & is domain required?

[Soulsender]

Hello, I have a couple questions I am having trouble with.

• Does .env need to have all fields filled in?
• Is a domain required, or are you able to set it to be a subdomain of a domain you already own?
  • If you can make it a subdomain - if your domain already has an SSL certificate, do you still need to get a new one with the .env file?

Appreciate anyone getting back to me, I'm having a hard time finding the answers to my questions.