I would use the link I gave you above for Munich transit instead of DB.

I use DB for intercity trains.

Here it is again:

https://www.mvv-muenchen.de/en/index.html

I usually just buy a ticket at the kiosk at the train station. It has a button for English. There is also an app for your phone.

You can put in the destination (Rosenheimerplatz) and it will calculate the fare and spit out the ticket once you put in your credit card. Or, you can choose a day ticket + airport and go anywhere you want in the city using any transit you want (bus, tram, Sbahn, Ubahn).