@Ritzy I don’t see any reason for loosing your German citizenship. If you already have the German citizenship you could acquire the British citizenship as well if you fulfill the criteria and have two citizenships. The transition period is the time period that currently is set so EU citizens that are already living or thinking of moving to the UK can take all necessary actions in order to remain valid in the UK or UK citizens that are living in the EU or thinking of moving to another EU country to do the same. Also during the transition period UK and EU will negotiate further relationships together like lets say trading deals etc.
Hope this helps