There’s no one standard…except for faxes.
HL7 and FHIR have been around for decades. Exchanging data is actually the easy part.
The problem is typically more on the business logic side of things. Good example is the fact that matching a patient to a particular record between facilities is a much harder problem than people realize because there are so many ways to implement patient identifiers differently and for whoever inputs a record to screw up entry. Another is the fact that sex/gender codes can be implemented wildly differently between facilities. Matching data between systems is the really hard part.
(I used to do HL7 integration, but have since moved more to the systems side of things).
That’s likely a peculiarity of the niche you’re in. HL7/FIHR are the norm for enterprise-level systems. Hospitals couldn’t function without it and at any given time we typically have multiple HL7 integration projects rolling just as a mid-size regional.
Definitely less defined in the small-practice and patient-side space. Though, like I said, the big problem there ends up being data normalization anyway.