Can I add other channels to my apple tv ie PBS, G

What is your model Apple TV? Identify your Apple TV model - Apple Support

Apple TV (4th generation or later) has an App Store where you can download more apps.

Apple TV (3rd generation or earlier) do not have an App Store, and the available services are fixed (per region).

an appletv is like an ipad or iphone that use your tv as a display so if your pbs has an app in app store you can download it and view their content