If you buy it via DB app it should work - like all other tickets in the DB navigator. If it is an eticket, you can use the native app “Pass Viewer” to open it.
No, it depends from the way the issuer distributes the ticket. I get mine from hvv via my employer, and there the variant for apple handies produces a downloadable pkpass file. Might be totally different for other issuers
Order the Deutschland-Ticket by your local Verkehrsverbund is always a good idea. Works great for me. Hint: You can buy the D-Ticket from your local Verkehrsverbund as a real physical card, this has also some pros. If you found a dealer for the card you can add a information about this dealer in this list. Hint #2: The dealer will get some money for every saled D-Ticket. Maybe you love to see this money by a local dealer?
I remember to read about some flaws about some other dealer but I forgot the details. Please check on your own. I found also a lawsuit against the DB app Navigator (see here)
I think @joboe said that either you buy it in db navigator (Android App) - then you can use it within the db navigator (Android App) OR you can buy it from some different issuer (e.g. local Verkehrsverbund) where it MIGHT be an eTicket which could be used via Pass Viewer (native app).
But for my ticket bought from hvv the eTicket could not be validated by ticket control despite being seemingly correctly displayed in Pass Viewer.
Personally, I would prefer any time to buy the ticket from my local Verkehrsverbund (and not hvv) as a physical card, but then I would not get reduced ticket price from my employer