How to change node add form
In drupal 8 there are new display modes. They are not only to view the node (view mode). They are also for the node add form (form mode).

For example to change the node add form of the content type article you go to:

/admin/structure/types/manage/article/form-display


Here you can select which fields you want to show and how they are formatted (field widgets). You can make more adjustments with php in form alter hooks or with twig in templates.

By default you have one form mode for each content type. You can create more if you go to the admin interface of display modes:

/admin/structure/display-modes


If you want to use different form modes for different users you can use this module:

Form mode control

Readme:


  WHAT FOR ?
  
  Form Mode Control allows you to use the form modes for any bundle /
  entity, per role and for edition / modification. Which means using
  different forms (fields, order, widgets, etc.) for different roles.
  
  HOW TO USE IT ?
  
  A - DEFAULT FORM MODES
  
  1 - You must at first add form mode for content entities. Ex. : go to
  "www.your-site.com/admin/structure/display-modes/form/add" to add a
  new form mode.
  
  2 - Activate the form mode for the related bundle of the right entity.
  Ex.if we want to activate a form mode named Super 2 (machine name =
  super_2) for an article (entity type : Node, Bundle : Article). Go to
  www.your-site.com/admin/structure/types/manage/article/form-display
  and activate it.
  
  3 - Go to www.your-site.com/admin/people/permissions, a section named
  Form modes control was added for all form modes activated (and only
  activated), So , configure all permissions and give roles permissions
  to access form modes. NB, the permission Access all form modes allow
  you to access to all form modes linked to bundle and entity type.
  
  4 -  Then configure on the Form mode control administration page
  (www.your-site.com/admin/structure/display-modes/form/config-form-modes)
  and give for each role a default form modes for creation / edition (of
  course, each role must have access to the form mode).    B - ACCESS
  DIRECTLY FORM MODES    You can also use it with a simple extra query in
  the URL (?display=machine_name_of_the_form_mode) if the role is
  allowed to see it. Of course, the user must have the right permission
  for it.        Ex. : if you want to access to super_2, go 
  www.your-site.com/node/add/article?display=super_2.