There are couple of approaches I know;
communication via typical REST. Straightforward. Better for non-real-time/non-frequent calls.
Remote Procedure Calls. Pretty solid approach but requires extra investment of effort for managing Schema Definitions. There are already very popular libraries like Google's gRPC, Apache Thrift etc. Sometimes, this approach may be an overkill if used for tiny usecases. Less overhead as it uses HTTP/2.
Cheers!