[Django]-How to identify and save the right "user" when we use Django (Existing proj) for authentication and FastAPI (new feature) for API's?

This is mostly a software architecture design problem, In your old login endpoint put every required data by the new FastAPI project in the JWT token payload(such as user_id, user_name and etc), sign it and give it to the user:

# using PyJWT
import jwt
user_token = jwt.encode(
     {"user_id": "1222", "user_name": "yosef", },
     "YOUR_SECRET_KEY",
     algorithm="HS256",
     headers={},
)

Afterwards in new FastAPI project endpoint, validate JWT token in request header with the key you have signed it in Django login endpoint:

# using PyJWT
import jwt

try:
    data = jwt.decode(JWT_STRING_VAR, "YOUR_SECRET_KEY", algorithms=["HS256"])
except jwt.ExpiredSignatureError:
    # reject
    ...

If the token is valid you accept the request and use the payload as confirmed data to do your usual endpoint functionality and if sign is not valid reject request.
This way you are not required to change your old endpoints at all.
*note: instead of PyJWT you can use SimpleJWT or any other package for Django and a suitable JWT package for FastAPI
**note: this scenario may go crazy complex when your JWT payload may get changed before the JWT expires, like when user_name changes but you can not change that in JWT and user can use it in your other service with old data, there are more solutions on this, like removing the properties that may change from payload and get them directly from database, meaning you query Django populated table from FastAPI, you can achieve this by import dynamic tables in SQLAlchemy
***note: system design schema image