Compliance is a handful of specific decisions rather than a badge. Encryption in transit and at rest. Every record access logged with who and when. Real authentication and session handling. A signed BAA with every vendor in the chain, hosting included. PHI mapped before anything is built, because retrofitting costs more than doing it.
For interoperability we work with HL7 v2, FHIR, CDA and CCD, and DICOM for imaging. Which one applies depends on what you are connecting to, and that is a discovery question rather than a checkbox.
If you intend to sell your system to providers who depend on federal incentive programmes, it needs ONC certification, which runs alongside development with its own cost and calendar. We ask which situation you are in during the first week, because the answer changes the architecture.