While you wait for more targeted answers, you might be interested in some threads from the FAQ section on “Operation Overlord aka D-Day:
German fortifications in Normandy at the start of the Invasion of Europe, 1944. - 11 comments, over 1 year old.
The original poster asks for books to read about the Atlantic Wall and commenters oblige them.
Why did the US naval bombardment before D-Day not destroy the German pillboxes above the beaches? - 18 comments, over 1 year old.
The commenters here note the inaccuracy of period weapons but mostly delve into the general preparations done by the German forces with their defenses which made them resistant to bombardment and air attacks.
How did the Allies "hide" all the ships from German reconnaissance? - 2 comments, over 7 months old.
Despite the title and lack of the comments, a commenter talks about the effects of attrition on German air forces in relation to their reconnaissance capabilities.
How much of a difference would the Panzers in France made during D-Day if they were put to use earlier? - 6 comments, over 2 years old.
The commenters discuss the various obstacles that the German's faced in using their armor to oppose Operation Overlord.
Did the Allied Forces have complete air superiority over Northern France during D-Day? - 53 comments, over 2 years old.
The commenters here give a rundown of the material, personnel, and logistical constraints that afflicted German forces on the eve of the invasion.
How much warning did Germany have of the D-Day invasion? - 11 comments, over 6 months old.
The commenters here give a breakdown of the German intelligence picture in the immediate runup to the invasion as well as a chronology and analysis of the actions of German commanders in France before and during the invasion, most notably Erwin Rommel.
Why did the Germans believe the Allied invasion would take place in Calais, and not Normandy? - 7 comments, over 7 months old.
This thread discusses various locations that the German's believed the Allies would land at due to both geography and the Allied counterintelligence campaign.