Module development and code questions

Hi Guys, 
I'm using xmlrpc_encode_request() this method in Drupal 7 and now I migrated my site to Drupal 9 and I want to use this method. How to use the same ? 
I'm using like this: 

I'm upgrading my theme and I use a combination of Paragraphs module and an Image Field. Previously, I had a function to get the default image. $field_name is the `field_image`:

  $field_definition = $variables['paragraph']->getFieldDefinition($field_name);
$uuid = $field_definition->getSetting('default_image')['uuid'];
$files = \Drupal::entityTypeManager()->getStorage('file')->loadByProperties(['uuid' => $uuid]);
if (count($files)) {
$file = reset($files);
return $file;
}

Hello,

I like to add a placeholder "Enter your name" to the "Your Name" text field of the feedback form using a twig template. 

Below worked for the email field. 

File Name: input--email.html.twig
Directory: themes/mytheme/templates/form

I've been working with address module (Version: 8.x-1.12) and I've been able to figure out how to override subdivisions based off of the example (https://docs.drupalcommerce.org/commerce2/developer-guide/customers/addr...) of adding the UK counties using GreatBritainEventSubscriberas a guide.

Hello everybody!

I need some inspiration/advice on which way to go further.

Site: a free site for shrinks. In return I want members to paste an iframe on their personal site (SEO-shizzles, you know)

Target : (authenticated) member has a personal iframe available (content includes their name and the link links to their personal page (view) on my site)

EDIT: Sorry if this is a simple question, I've been out of Drupal since D7 !!

Hello. 

I need hook/s for create an custom module for possibility for user/s for autologin.

This user/s only may see only one view or type of content.

Only for view content and view of thid content type.

With possibility of creation QR code for login. 

Not other permissions. 

Thanks