> In this case I doubt they're networked apps so they probably don't have a server API.
I think it would be very unusual this decade for software used to run either a medical practice or tax accountants to not be networked. Most such practices have multiple doctors/accountants, each with their individual computer, and they want to be able to share files, so that if your doctor/accountant is away their colleague can attend to you. Managing backups/security/etc is all a lot easier when the data is stored in a central server (whether in the cloud or a closet) than on individual client machines.
Just because it is a fat client MFC-based Windows app doesn’t mean the data has to be stored locally. DCOM has been a thing since 1996.