Publish dynamic pages? I want to create a website with form, where users can insert the information in the form and then on form submit, information goes into a database table and also goes to the third party API.

Administrators can modify the specific record and add more information, and then publish this record as a web page with a unique URL.

Is it possible in Drupal 8?
How can I achieve that? Is there any specific module I need to use?
I'm not sure if in D8 webform submissions are useable, fieldable entities yet.

Although I believe the EForm module may work as you expect. Allowing you to define forms the public can fill out and then very easily turn the content into drupal "pages" or reachable content at some url pathauto pattern.

Allowing the public and administrators to modified different fields, or subsequently edit submitted data is a Roles/Permission issue that drupal Core or other modules such as Field Permissions will let you achieve.