For the Cardiogram app, most of the UI is developed in React, Redux, and D3 (for interactive charts). For functionality that needs to be native--for example, integrating with the Health app or Google Fit--we use Swift (for both iOS and watchOS) or Java (for both Android and WearOS by Google, which is itself based on Android).

The backend is written in Node with PostgreSQL as the database. Since it's a large dataset (terabytes), we rely quite a bit on PostgreSQL 10's partitioning feature for our larger tables: https://www.postgresql.org/docs/10/static/ddl-partitioning.h...

The machine learning code is written in Python, using Keras, TensorFlow, Pandas, scikit-learn, statsmodels. Our AAAI-2018 paper has a bit more info on how DeepHeart (our deep neural network) works. We also do feature engineering and traditional machine learning models when appropriate: https://www.aaai.org/ocs/index.php/AAAI/AAAI18/paper/view/16...