Nested documents

The schema ia already prepared for nested documents.
But there's no integration for Search API yet. So you need to implement the required processor, datasource and Event subscribers by yourself.
Ideally you contribute them.

Thanks @mkalkbrenner
I am new to this, can you provide some example/code snippet to begin with?

You can look at the source code of this module and Search API Solr itself.
For example it would make sense to optionally store entity references as nested documents.
But I don't think that this is a beginner task and I can't point you to any example.

We are using the https://www.drupal.org/project/file_entity → module to get nested Files indexed on search_api.
This is used for files inside Drupal of course.

Nesting something in drupal entity field is something different the nested documents in Solr.
Or did you write custom code to index that data as a nested document?