Apple announced the new feature months ago, but now we know how Health Records will work. Predictably, it's located within the Health app for easy access. Ideally this keeps the info handy for the patient to view and hand over to doctors -- which is easier than carrying around an entire medical file. All data contained will be encrypted and protected with the user's iPhone passcode.
Patients from nearly 40 health institutions across the US (listed here) can view their records after updating to iOS 11.3, but it wouldn't be surprising if that list continued to grow.