No app for the computer?

Computers are not like phones and tablets, although are similar
Apps are created for mobile devices to add functionality whilst overcoming the problem with small touch screens
So many apps do not have their equivalent application on a computer

Many apps are web based (eg PayPal), ie you use the app to access a web service, the work is not done on your phone, it is done online
The app is simply the doorway to the web service

If there is no app for the computer
You access the service by going to the appropriate website
Eg for PayPal
Open your web browser (ms Edge, G Chrome etc)
Type in the address in the address bar along the top (or search; Google it)

In this example
Type in
Sign in with your email address and password (as you set up). Done, you’re in