I've never understood how any pre landing efforts using planes or even the navy from a distance could lead so much German residence left as to make the actual beach landings a slaughter. please educate me.
There are a few reasons why the D-Day aerial bombardment failed to do any damage- the most impactful is simply that it missed. Ostensibly, the impact of the bombardment was to be twofold- it was supposed to destroy the fortifications overlooking the beach and cause disarray among the ranks of the defenders. There is a problem with this, however, because at the insistence of Lieutenant General Omar Bradley, commander of the US First Army, the heavy bombers were to be equipped with light bombs. Bradley was concerned about cratering on the beach, and for good reason. There were only five exits leading off Omaha Beach, and of these only four were capable of being traversed by vehicles without significant improvement- thus movement off the beach was already constricted. If the worst case scenario outside of the landing failing outright occurred, and the assault forces were unable to expand the beachhead to a point where German artillery could not reach the beach, extensive cratering of the beach would have wide-ranging effects. Repairing the cratering and making the beach suitable for traffic would require concerted efforts by engineers, men who already had their plates full removing obstacles and minefields. In the interim vehicles, supplies, and heavy equipment would be all but immobilized on the beach, unable to be dispersed. The concentration of all this materiel would present quite the target for German gunners, and its destruction would grind to a half the advance of frontline units for lack of supplies. In this fashion, in the worst case scenario a vicious cycle is born- the engineers are unable to accomplish any of their tasks in a timely manner, slowing movement off the beach; the lifeblood of the formations ashore is concentrated in one spot, unable to be dispersed; and it is destroyed by German artillery fire, which, as an aside, also kills engineers working to improve the beach while damaging their equipment and shattering work they’d already accomplished, thus slowing the whole process down even more. This higher-level concern was a valid one, and it won out over the actual destruction of the concrete emplacements overlooking the beach. Therefore, even if the bombardment had hit Omaha Beach, the physical destruction would have been limited to damage to field fortifications, disruption of communications, and demoralization of enemy troops.
In any event, however, the bombardment missed altogether. The morning of 6 June was overcast, and the heavies were forced to rely on instruments to “see” through the clouds. In the event of poor weather the bombers had been told to delay their drop by up to 30 seconds so as to avoid hitting the assault troops, who when the bombers came over were in their landing craft about 2 miles offshore. This is exactly what happened- the weather was poor and the bomber crews followed the instructions they had been given, with the result that all of the bombs fell inland of the bluffs overlooking Omaha (on which the defenses were arrayed) and doing no damage to the defenders.
Planning Operation Overlord required quite a bit of subterfuge. If the Allies made any clear moves that the reconquista of Western Europe would begin on the beaches of Normandy, the Wehrmacht and Luftwaffe would have moved their divisions and squadrons to bombard a narrow beach front, and the landing would have been impossible.
If the whole of Nazi Germany's available Western European forces were centered on the landing beaches, Overlord's breakthrough phase was unlikely to succeed. A significant factor in the success of Operation Overlord and the often overlooked Operation Dragoon (the invasion of southern France in mid-august 1944) were the allied attempts at disinformation. Because allied counter-intelligence operations made convincing cases for invasion from Norway to Bordeaux, Nazi Germany was forced to spread its occupation forces across thousands of miles. When the Allies finally struck in June 1944, German forces were too disparate to break the Allied beachhead in any given zone.
Considering the technical limits of aerial bombardment, no fielded weapon in 1944 was capable of obliterating coastal artillery in concrete bunkers. The general counter-blitz against German aircraft production did ensure, however, that the Luftwaffe had its war potential in central Germany instead of France. This made the landings (and their heavy close air support) possible.
There's always more that can be said but I thought you might enjoy some of these older threads from /u/thefourthmaninaboat on a similar topic.
On the general ineffectiveness of bombardment, albeit from a naval side, is How effective were Naval Bombardments during World War II?