Modify existing custom content form
Thank you very much! 
I really didn't need routing! 

Well, finally I found the solution to my problem - I created the module with 2 files only - mymodule.info.yml and mymodule.module.
In .module file I only added these lines of code and everything worked! (Almost)

function mymodule_inline_entity_form_entity_form_alter(&amp;$entity_form, &amp;$form_state) { 
if ($entity_form['#entity_type'] == 'node' &amp;&amp; $entity_form['#bundle'] == 'my_custom_content_type_machine_name') { 
  unset($entity_form['field_I_need_to_hide']); 
}}