Jesse Chen on the Engineering at Meta blog:
Instagram uses Python (Django) for its backend. By using the same backend for Threads, we could leverage a lot of the existing tech stack for Threads and reuse most of our existing data models, business logic, security features, and server infrastructure. This also meant users could sign in to the app with their existing Instagram account, making it super simple to onboard and set up your Threads app.
The Threads mobile apps themselves were built primarily with Swift on iOS, and Jetpack Compose on Android.
Always interesting to get a peek behind the curtain of large proprietary systems. I had no idea Instagram and Threads were built with Python.