EPiServer Find is built using ElasticSearch which is well documented in the ElasticSearch guide. However, EPiServer Find enforces a number of conventions to be aware of. The EPiServer Find API also restricts some functionalities such as adding mappings (which is automatically handled using the conventions), and adding and removing indexes.
Other client integrations
EPiServer Find is by no means limited to being used from .NET. While we do add additional functionality and enforce some restrictions and conventions, many of the client APIs and integrations that exist for ElasticSearch will also work with EPiServer Find.