Hey Kyra,
You must pay taxes from your developer proceeds in countries where you are a tax resident in a given year. Different countries have various criteria for being a tax resident. If you are based in the US and haven't lived anywhere else, you only have to worry about the US taxes. Furthermore, you don't sell any subscriptions personally. You designate that to Apple since it acts as your agent. And thus, the company handles the taxes in all the countries where your app is for sale. For more information, please read the Apple Developer Program License Agreement.
And yes, you can choose where your app is available in App Store Connect. You can limit the availability of an app itself, purchases, subscriptions, or even promotional offers.