Send user auto generated password on different email

On one of my websites, I'm trying to customize different email content like user activation email/new user registration email.

Basically, on the activation email, it sends an activation link with the username an auto-generated password but I want to alter the functionality a little bit.

On the activation email: it will send the activation link, nothing else. On the next email/registration email: it will send the username, auto-generated password etc. I want to move the auto-generated password on the second email instead of the first email.

I placed $user_pass on the second email but that doesn't work.

Hopefully, that is understandable. Any kind of guideline or help will be highly appreciated. Thanks in advance