Where is the core implementation for Magento 2 Rest API fields=items[] filter to select specific values from the result?

I am trying to figure out where is the below rest API request fields=items[] filter core module implementation? I still couldn't find it out. Did anyone notice it?

GET <host>/rest/V1/orders?
searchCriteria[filter_groups][0][filters][0][field]=status&
searchCriteria[filter_groups][0][filters][0][value]=pending&
searchCriteria[sortOrders][0][field]=increment_id&
fields=items[increment_id,entity_id]

Reference: https://devdocs.magento.com/guides/v2.3/rest/performing-searches.html

I look forward to hearing from you.

Thanks

Dasitha