Hello @Willias4 and welcome to the AT&T Community Forums!
I am sorry to learn you are having trouble to understand how your wireless plan can be affected by Google Chromecast. I can help you with that.
AT&T will not meter, nor charge for your Wi-Fi data usage.
Chromecast is a connectivity device which act as a bridge between your smartphone and your TV or speaker(s). In order to transmit audio/video from the smartphone to the Chromecast unit, both must be connected to the same Wi-Fi network. And, there you have the bridge.
The app originating the audio/video that your smartphone is transmitting to your Chromecast may need an internet connection to acquire its content.
For example, YouTube. A YouTube video may be free, as well as using the Wi-Fi bridge to transmit it to the Chromecast unit. But YouTube must be connected to the internet in order to stream the video into your smartphone, which will be later transmitted to your Chromecast unit.
If YouTube was connected to your AT&T Wireless data plan at the time of being connected to your Chromecast, the data used by YouTube will be charged against your data plan.
If you have an iPhone, and the Wi-Fi Assist feature was enabled, even when connected to Wi-Fi, the smarphone would have been able to decide to automatically connect to your data plan if the Wi-Fi network speed was not fast enough for streaming the audio/video. Android has a similar feature called Auto Network Switch.
Misael Bilingual CS&SR I AT&T | Mobility
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