Apple TV+ (the subscription) - how can I tell what it actually offers?
The easiest way I browse TV+ content is by opening the TV app on iPad and look for the channels. Tap on it, then you can see all the major shows. Also, on each show in the app, look for the service in the text as well as the badging in the lower right corner.

Of course the free episodes can and will change, so I wouldn’t count on things that are free today being so in a month or vice-versa. If you prefer web access: https://tv.apple.com lets you browse and watch off device (AppleTV, iPhone, iPad, etc..) in a native app.
Since the native apps show all channels, not everything is included nor is everything in the app advertisement free, however what you pay for from the + is commercial free, downloadable, playable for the duration of the subscription whether it’s subsidized or paid in cash / store credit.