Short answers, pulled from the story.
Roman authors like Tacitus wrote in 98 AD that the term might have begun as the label for just one small tribe called the Tungri. Julius Caesar first used the term during his Gallic Wars in the 1st century BC when he encountered peoples beyond the river. Modern researchers debate whether Celtic, Germanic, Illyrian or Latin roots explain the original meaning.
Ancient geographers mapped Magna Germania stretching from the Rhine River in the west to the Vistula River in the east. Roman descriptions placed the southern boundary at the Danube and extended northward into known parts of southern Scandinavia. This vast region spanned what is now the Netherlands, Poland, Denmark, Norway, Sweden and Germany.
Arminius defeated four Roman legions at the Battle of the Teutoburg Forest in 9 AD ending imperial ambitions beyond the Rhine. This victory dissuaded Rome from conquering Magna Germania permanently and made the Rhine the permanent border between empire and free territory. Four legions and the Classis Germanica fleet later guarded the western provinces known as Roman Germania.
Magna Germania corresponds archaeologically to the Roman Iron Age spanning from the 1st to the 4th century AD. Excavations reveal largely agrarian societies lacking cities or independent coinage systems that distinguished them from Roman territories. Archaeological finds show flourishing trade networks where amber flowed out of Magna Germania while luxury goods entered on a large scale.
Diocletian renamed Germania Superior to Germania Secunda and Germania Inferior to Maxima Sequanorum during his reforms in the 3rd century AD. Flavian dynasty leaders created Germania Inferior and Germania Superior in the late 1st century AD from parts of Roman Gaul. The Crisis of the Third Century saw Alemanni and Frankish confederations emerge in Magna Germania to raid Roman holdings.
A large number of people fleeing the Huns crossed the Rhine from Magna Germania into Roman territory in 406 AD. This mass movement led to the eventual collapse of Roman rule and emigration of many Roman elites from the provinces. Germanic peoples including Alemanni and Franks subsequently occupied former Roman lands throughout the 5th century AD.