How did Germanic peoples come to replace Celtic peoples in mainland central Europe?
Not to discourage any further answers but you should read these older answers on the topic.
What caused the Celtic expansions across Europe, and then the later Germanic expansions? Were there any shared causes? by /u/Miles_Sine_Castrum
Who were the ancient "Celts" that lived throughout Europe during the time of the Roman Empire? Why is it today anything "Celtic" usually always refers to only Ireland? by /u/Daeres
If the Celts didn't exist, what ethnicities existed in the areas that spoke Celtic languages?