How and why did the Germanic people’s expand into present-day Germany, where the Celts were previously living? Why were they attracted to the area, and did they have any superiorities in areas like warfare that enabled them to do so?
More can be said, but you might find elements of answer in these previous posts :
Briefly, while Celtic peoples (as Eburones or Helvetii) moved in Gaul by the IInd and Ist centuries BCE along with migration of peoples from North Germania, onomastics, genetic and archeological evidence point to continuity in Rhenan and Danubian Germania populations.
Reasons for migration of North Sea/Baltic Sea peoples towards the Alpine Arc can be tied to those of La Tenian peoples of southern Germania (whose prestigious influence over northern cultures was significant), moving southwards in search of better farmland, access to Mediterranean goods, military employ, etc. and quite possibly involving Germanic-speaking population with them while the famous migration of Teutonii, Cimbrii and Ambrones likely involved as much Celtic-speaking than Germanic-speaking populations for instance.
The decline of Danubian societies (maybe due to social upheaval, consequences of warfare and raiding, but as well of Roman conquest) seem to have been followed by their cultural and social 'germanization', new elites replacing the old or, at least in many cases, intermingling with them (hence the likely Celtic roots of several Germanic onomastics in the early Roman Iron Age) reinforced by both Roman expectation on Germania and local population self-identity echoing these.