I tried to follw the example of https://doc.ezplatform.com/en/2.5/guide/solr/#extending-the-solr-search-engine-bundle and ran into an error, saying there is no field "fulltext_fulltext" in solr.

I took the schema.xml from vendor/ezsystems/ezplatform-solr-search-engine/lib/Resources/config/solr/schema.xml which indeed does not contain a dynamic field "*_fulltext".

In vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/Search/Common/FieldNameGenerator.php there is a fieldNameMapping from "ez_fulltext" to "fulltext". There are even some more mappings, not existing in schema.xml (e.g. *_doc and *_h)