Blockchain technology offers a decentralized database that enables the registration and maintenance of electronic health records (EHRs) through the implementation of encryption policies to ensure privacy. The inherent properties of decentralization and immutability in blockchains make them a practical choice for serving as a database for recording EHRs. Given that EHR files are typically generated by various entities such as hospitals, laboratories, clinics, and mobile applications, it is possible to store each set of collected data in separate blockchains adhering to different standards. However, due to the requirement for real-time access to medical records and concerns surrounding confidentiality and privacy, it is imperative that EHRs registered on different platforms are able to interact with one another online. Therefore, Interoperability becomes a fundamental necessity for blockchain-based EHR systems. This paper aims to survey existing research on the implementation of Interoperability in EHRs using blockchain technology. The study's findings suggest that achieving a comprehensive solution involves ensuring that all stakeholders follow standardized EHR protocols when recording information. Furthermore, it is advised to use cloud databases for storing large-scale EHR data, while limiting blockchain data storage to identity information and maintaining the integrity of cloud-stored data. To effectively enforce these processes, blockchain smart contracts are utilized. By employing these mechanisms, blockchain can serve as a suitable platform for recording and maintaining interoperable EHRs. Additionally, a detailed multi-layer software architecture is a common practice in the field, even though there is no consensus on the role of 3rd party auditors in it.