If you go to Manage | Database | Relationships and double click a relationship line between observations and one of the other Tutorial: What are Table Occurrences?, you will then be able to go back to this field and enter data in fields from that table occurrence. You can, of course, repeat this for each of the other table occurrences.
You appear to be setting up one to many relationships where one observation record may be linked to multiple related records in each of the related tables. Your layout design will only permit you to create a single related record in each of the related tables and enter data into only the fields of that single related record.
You'll need a different layout design if you want to create a group of records from the same table that are all linked to the current observation record. The most commonly used layout object for that is a Portal and you can look up portal's in FileMaker Help and any training materials you may have to learn more about them.