Official pages can be found here in French [0] and German [1].
I found the conditions as described in the DW article not obvious, so here is an attempt at stating them more clearly:
In each country, there are 30k free tickets on a first come, first serve basis for people 18 to 27 years old.
Going forth and back multiple times between the countries is not intended, people therefore need to plan a single journey with up to seven days of travelling (in the sense of "using public transportation") in the other country. People can choose these days freely. The days also do not need to be consecutive but first and last day of the journey must be within one month from each other (while 1st of July is the earliest possible date for departure and 31st of December the latest for returning). They also need to actually depart their home country on the first day and to return to it on the last day that they chose.
These rules seem to be in place to prevent some forms of abuse of the ticket.
Only available for people who sell themselves to Google or pay the Apple premium. Well, one could say it's a gift so don't complain. No, it's paid by the taxpayers. And French/German taxpayer money should not be used to make citizens less free for the benefit of a US duopoly which has continuous problems to comply with EU legislation.
P.S. I am no longer affected. I benefited from a French/German youth program 45 years ago, which I am grateful for.
Using the ticket requires to use a phone with either Google Play or an iPhone. Alternative phone firmware (degoogled Android, SailfishOS, etc.), paper tickets, smart cards or anything else are not supported.
From the seller? It wasn't when I checked many months ago, when they had special Interrail campaign with flexible dates for app users but only fixed dates for paper tickets.
Of course there are unofficial sites. But firstly I would not trust that their APKs are unmodified and secondly I would not rely on it that it works with a ticket for 300 Euros or so. Some tamper/rooting detection might still trigger in an alternative firmware. Well, the youth ticket doesn't cost anything, but see above why that's not an excuse.
I found the conditions as described in the DW article not obvious, so here is an attempt at stating them more clearly:
In each country, there are 30k free tickets on a first come, first serve basis for people 18 to 27 years old.
Going forth and back multiple times between the countries is not intended, people therefore need to plan a single journey with up to seven days of travelling (in the sense of "using public transportation") in the other country. People can choose these days freely. The days also do not need to be consecutive but first and last day of the journey must be within one month from each other (while 1st of July is the earliest possible date for departure and 31st of December the latest for returning). They also need to actually depart their home country on the first day and to return to it on the last day that they chose.
These rules seem to be in place to prevent some forms of abuse of the ticket.
[0] https://www.passefranceallemagne.fr/
[1] https://www.deutsch-franzoesischer-freundschaftspass.de