How can I use a view on site2 that consumes data from site1?
The External Entities module can help you in this task.


  This module allow you to connect to datasets from external databases
  and use it in your Drupal 8 website. While content lives external,
  Drupal sees those remote entities as internal. This makes it possible
  to alter the field displays, add references, comments, pathaliases,
  share buttons, webshop products and more.


You can read to Drupalize remote data with external entities.