We often come across use cases around pre-filling and the submission of data using AEM Forms from/to external databases (DBs). Out of many possible solutions possible to address such use-cases, one solid contender is a modular services based approach using AEM Forms in conjunction with Adobe I/O Runtime.
Adobe I/O Runtime is a serverless architecture with the benefits of flexibility, scalability, reduced operating cost, etc. Modular services deployed on Adobe I/O Runtime communicates with an external DB and processes the data retrieved from DB thereafter.
Meanwhile, AEM Forms contains powerful authoring capabilities. It has an intuitive UI that saves time in designing and publishing the form by reducing custom development effort.