customization – No plugin populate user information in to form

This is very simple, I am assuming you are using third party forms, if yes
Then you have to use Javascript and PHP.
You can paste this code in shortcode or you have to use hooks, as you are learning please go through this Link

PHP CODE

if ( is_user_logged_in() ) 
{
    
  $current_user = wp_get_current_user();
  $email = $current_user->user_email
}

To understand this method kindly follow this link

Wordpreess wo_get_cuurent_user()

Now as you have $email you can save that $email to Javascript something like this

<script>var email = <?PHP echo $email ?></script>

Now using this var you can populate these user details to the form.

If you face any issue let me know.