Apple TV Streaming Over Local Network

I've had some concerns that my Apple TV is streaming Plex content over the internet and not my local network (hitting Comcast monthly limit often)

I've been digging on the forums and see that you do need to have an internet connection and sign in to access your Plex Sever even if it is on the same local network.

I can't find any info on if the media is streamed locally or not.

Thanks