Module development and code questions

Hi

I have few forms build as below:

I'm using the RESTful API to create and update users.  I don't have a way to take an email address and convert it to a user number though.  I'm doing it now talking to the database directly but I don't want to use that method anymore.  Is there a way to do this with any of the RESTful APIs?  Or does it really just not exist?

Thanks

Marc

I am using the group module in my site then installed webform_node and group_webform to be able to add my webform submissions to a group, but I want to add some submissions to their groups programmatically like I am doing with simple nodes.

In hook_ENTITY_TYPE_insert() I add nodes to their group, I want to do the same with webform but not sure if it can be simple since it needed integration module to work with group module.

I have some nodes that aren't visible to the non-identified user because it is specified in the view that I´ve created.

However, If you know the path of the node , it´s possible to access to this. I would like that a non-identified user can't see the node although he has the path.

Hi, I am new to here. I am creating a quotes site which has a quotes custom content type. It has basically 3 fields: quote_body, author, tags(tag will be separated by comma and there are min. 5 tags for each quote). I am in search of a module that will help me to create a views page for each author or tags, I know I can do it manually with views for each author and tag, but since there tens of thousands of them, I can't do it that way, so I am asking if there any module that I can turn to? 

Thank you very much in advance. 

Is there any module for messages between users in Drupal 8 or 9. Actually I want to make this with Drupal core without module. Is this possible. Thanks for support