D365 Online out of box index building feature and index customization for custom entities

Hey Dynamics community.

Currently, I’m deeply involved in plugin development for the Microsoft Dynamics 365 Online platform. It widely uses the search queries so the search experience dramatically depends on search query overall performance. Thus, the search speed is crucial to use successfully the plugin.

Due to the lack of direct control under database, I’m looking for options to improve the search speed performance by creating indexes for both standard and custom entities. Otherwise, I generally ask if there’s any out of box feature to build indexes for entities. Questions:

1. How does it build an index for a standard entity (like email or task)? How can I trigger index (re)building for particular entity/fields? Is there an option to do that programmatically (through API)?
2. Does this build an index for a custom entity/field (even in case it's derived from a custom entity)?
3. What are timeframe and requirements to build an index?
4. Does the trial version of D365 support indexing?

Thank you for any help/assistance to answer this question.