You can do it only if the website gives you API access.
In short, an API is a list of functions that the website gives programmer so they can interact with the data available behind it.
In other word, you can only access "programmatically" whatever the site owner allows you to (super fucked up, I know but the web in its current state is
fundamentally broken).
There are ways to circumvent this of course, (to a certain extent). But they are technically difficult to achieve and will probably be rejected by the app-store owner (the new dictators of the web...).