Metro and bus app

There are only three metro lines in Athens, with a map you can easily manage your metro trips without any application.

When I'm in Athens I use the OASA Telematics app, especially for buses, trolleybuses and trams (it shows the bus location in real time and finds the best route from one place to another) This is the official public transport app, useful if you get used to it. It is free, in English and Greek.

But Google Maps also works well although I noticed a few small errors.

Without an application, OpenStreetMap with the transport layer is very convenient (and up to date) for visualizing all transport lines

https://www.openstreetmap.org/search?query=athens#map=11/37.9757/23.7348&layers=T

Citymapper covers Athens. Its generally the best transit app you'll find covering multiple cities.