I’m not sure what you mean by “use that app” in relation to OV Pay. OV Pay is the name of the nationwide system that allows you to pay for public transport by tapping your credit or debit card against a reader in order to pay for public transport. Instead of a physical credit card, you can also tap your smartphone, Apple Watch, Google Watch or other device at the reader, as long as the wallet on that device is linked to a debit or credit card. OV Pay doesn’t require an app.
I will link to the website again, where you can read how it works https://www.ovpay.nl/en/how-it-works
As mentioned by another poster, in order to get the senior discount on the buses, trams and metros, you’ll need to get a personal OV chipcard linked to a European bank account. Technically, tourists with a European bank account can apply for a personalized OV-chipcard, but it’s a lot of hassle and it costs €7,50. So it’s only worth it if you make several trips or if you use buses and trams a lot. This discount does not apply to trains.
If you use your credit card to tap in at Schiphol and tap out at Haarlem train station, the full fare for the bus ride is €5,49.
Metros don’t run between Haarlem and Amsterdam, so getting a metro pass to travel between these two cities won’t work.
If you use OV Pay, the fare for the train is €4,90. If you buy a ticket at the train station, you need to pay €1 extra to cover the cost of the single use disposable chipcard the train ticket will be supplied on.
So to wrap up; I think that the easiest thing for both your bus ride from Schiphol to Haarlem and the train from Haarlem to Amsterdam is simply use your credit card to tap in and out.
If you’re still unsure about how this works, perhaps you can ask your friend who lives here to show you?