Greetings! How fortuituous that I just finished a fairly comprehensive overview of the Wehrmacht (the German armed forces during World War 2) in this thread here (shameless plug there, worth a read if you are interested. Some of this response (mainly the bits about German troop sizes) has been taken from that as well, but I have added on some more specifics about the Polish opposition facing them. Let's begin.

Adapted from linked response:

The Polish Army in September 1939 consisted of thirty regular infantry divisions, ten reserve divisions, eleven brigades of horsed cavalry and one armored brigade. Facing them from Germany and East Prussia were fifty-five German divisions. Twenty-four of them were infantry divisions in the so-called “first-wave”, regular soldiers and the best reservists. Fifteen were infantry of regular cadres and younger reservists, whilst the sixteen divisions were composed of six new Panzergruppen and ten fully mechanized infantry. Importantly however, the Germans had overwhelming superiority in the air, with over 2,000 modern aircraft crewed by well-trained Luftwaffe pilots decimating the 600 obsolete Polish aircraft.

Unfortunately, my knowledge does not extend so far as to the general makeup or size of German vs. Polish divisions, so hopefully another contributor with past experience in this field can comment on the breakdown of what an actual infantry or panzer division consisted of. What I can do however, is offer a general overview of the actual tactics the Germans employed.

The Wehrmacht plan for Poland was formed around two key "army groups". The first was Heeresgruppe Nord (Army Group North), under the command of Generaloberst (roughly the equivalent of a colonel-general) Fedor von Bock. Its 3rd Army was based in East Prussia, and its 4th Army in Eastern Pomerania. The 4th Army was to link up with the 3rd Army via the Danzig Corridor, and then both would attack along the line of the Vistula-Narew rivers, aiming for the capital of Warsaw.

In the South, Heeresgruppe Sud (Army Group South) under the command of generaloberst Gerd von Rundstedt, was to advance through southern Poland towards Warsaw as well, encircling Polish resistance along the way. This task was entrusted to its 8th, 10th, and 14th Armies.

The Wehrmacht were assisted in their task on the 17th of September, when the Red Army invaded Poland from the east. Two weeks into the campaign, and most of the Polish divisions had been encircled or fragmented into smaller forces. Pockets formed around the cities of Kutno, Radom, and Warsaw, with the capital itself falling on the 27th of September (though resistance did continue until as late as October 6th.

With that, the Polish campaign came to a swift end, and the world was shocked at the sheer speed of the German advance. In the coming months, years, and decades, this speed would be attributed to a "new" form of warfare: Blitzkrieg (lightning war). I would like to note a few things here:

Without going into too much of the debate, the essence of it all is that Blitzkrieg was not the concerted effort of mechanized troops and Panzers which is often pictured when hearing the word. Instead, Blitzkrieg (if one can even use the term) referred to the Germany Army’s preference for fast, and short campaigns, which during the Second World War was launched though the use of Panzergruppen and the Luftwaffe’s “Flying Artillery”. In truth, the German campaign in Poland was not spearheaded by the lightning-fast and precise Blitzkrieg strikes which is often detailed. Instead, the Panzers were deployed in a dispersed manner, aiming to overrun key communication points, supply lines, and defenses.

I hope this response helps shed some light on your question, and feel free to ask any further follow-ups!

Sources

Balsamo, Larry T. "Germany's Armed Forces in the Second World War: Manpower, Armaments, and Supply." The History Teacher 24, no. 3 (1991): 263-77. https://www.jstor.org/stable/494616.

Chant, Christopher. Warfare and the Third Reich. New York: Barnes & Noble Books, 2000.

Fritz, Stephen. The First Soldier : Hitler As Military Leader. New Haven: Yale University Press, 2018.

Harris, J.P. "The Myth of Blitzkrieg." War in History 2, no. 3 (1995): 335-52. Accessed January 19, 2021. http://www.jstor.org/stable/26004454.

Parker, Robert Alexander Clarke. The Second World War: A Short History. Oxford: Oxford University Press, 2001.