Several services are built upon OTP's routing and transit data indexing engines. They expose these APIs:
The GTFS GraphQL API has been used by the Digitransit and otp-react-redux projects as a general purpose routing and transit data API in production for many years. If your input data is mostly GTFS then this is probably the best choice as it uses the same vocabulary.
The Transmodel GraphQL API is used at Entur in production since 2020. Like the GTFS GraphQL API it is also a general purpose API. If your input data is mostly NeTeX then you might want to investigate this API as it uses the Transmodel vocabulary to describe its entities.
The Vector tiles API is a special purpose API for displaying entities on a vector map.
The Actuator API provides endpoints for checking the health status of the OTP instance and reading live application metrics.
The Geocoder API allows you to geocode street corners and stop names.
Legacy APIs (to be removed)
The OTP REST API used to power many apps and frontends. For years it was the only way to access OTP programmatically.
Over time it has been replaced by the GraphQL APIs and is scheduled to be disabled by default and eventually removed completely. It's therefore not recommended to use it.