React.js for the UI and state management.
Vercel for serverless hosting and easy deployments
FFMPEG for stitching
Node.js on the backend handling the rendering
Prisma 2 as a ORM for database interaction
GraphQL to perform operations on the data
This took me longer than anticipated about 11 months.
Let me know if you have any more questions happy to answer. You can DM me on twitter @michaelaubry