> When you declare a path operation function with normal def instead of async def, it is run in an external threadpool that is then awaited, instead of being called directly (as it would block the server).

https://fastapi.tiangolo.com/async/#path-operation-functions

OP either meant this, or its variation, such as async_to_sync and sync_to_async. https://github.com/django/asgiref/blob/main/asgiref/sync.py

Ofc this is a python example. I have no idea how it works in different languages.