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 functionality, such as adding mappings (which is automatically handled by the conventions) and adding and removing indexes.
Other client integrations
EPiServer Find is not limited to being used from .NET. While we add functionality and enforce some restrictions and conventions, many ElasticSearch client APIs and integrations also work with EPiServer Find. The ElasticSearch guide has a list of most of the projects.