You cannot search one post type by content contained in another post type. The only thing that ACF stores for a Post Object field or Relationship field is the post ID or array of post IDs for the post or posts that are selected. So the only thing you can search by it the ID of the post. How the query would need to be build would depend on what type of field you are using and whether or not is stores a single value or an array of values.
This is a guide that shows how to search based on an array in a relationship field
https://www.advancedcustomfields.com/resources/querying-relationship-fields/
