Camera not available for my macbook pro 2013

Hi there, I need help with regards to my macbook pro 2013 camera. It's not turning on when I open facetime, photobooth, zoom etc. It says, "facetime has not received any video from the connected camera...." in other apps it says, "camera not available."

Here's what I did:

1. Reset SMC,  NVRAM or PRAM
2. Use another account
3. Clean install - I performed clean install from Catalina downgrade to High Sierra, then going back to Catalina nothing happened.
4. I bought an external webcam - it did work once after a few tries.

Now, I have no more clues what to do. A bit frustrated because I need the camera for my online teaching. I hope there's someone out there who can help me. Thank you.