Berlin Brandenburg airport has a train station (suburban and regional trains) but also bus connections. Interrail would be valid on the trains but it’s not worth using a travel day just for this one trip.

What works best depends on where you’re going. Berlin is quite large and "city centre” is not very precise. Check the VBB website for transport in and around Berlin.

Prague airport has a number of options. I suppose that again depends on where you’re coming from.

PRaha: ONly normal option for about any newbee tourist is the 119 ( or so) BUS to nearest Metro stop.

There is a DPP=Praha transit, info office in each of the 2 terminals and it can arrange the various daypasses and other tickets and has info.

There is a giant and very well stocked ditto INfo office in the ticket hall of Praha HN=main station. Excellent free maps, in fact probably to big and daunting for newbees only visiting centre.

It makes no sense to spend a travelday that cost around 40-50€ for a trip that cost maybe 3€ normal.

DPP is FREE for all EU-citizens from 70.

Local transportartion INside the CZ is just 25% of the already very low normal price for junior and student till 26, ISIC must be shown. But that is not for local urban transit.

There is a site that overviews all transit options for all main airports, like getting to/fro or so. Just google.