Been there, done that:
Let me tell you something about those "40.000 planes build in Germany in 1944". First, here are the "Serviceable Aircraft Strengths of the Luftwaffe on the Eve of Overlord, 31 May 44". Again, this is the official OoB of the Luftwaffe:
And here is the "Serviceable Aircraft Strengths of the Luftwaffe on 10 January 1944":
Have another source: "Strategy for Defeat, The Luftwaffe 1939-1945" by Williamson Murray.
Don't confuse "Aircraft produced" with "Serviceable Aircraft". Now, how to explain the
astonishing difference between 40.000 planes produced and 5.000 planes in service (note that the data in the pic above is not the
serviceable "German Aircraft Strength", which explains the difference between the two sources.).
I cannot summarize it better than the
United States Strategic Bombing Survey (bolding is mine):
"German fighter production continued to increase during the summer of 1944, and acceptances reached a peak of 3,375 in September. Although it has studied the problem with considerable care, the Survey has no clear answer as to what happened to these planes; the differences of opinion between German air generals, it might be added, are at least as great as between those who have searched for the explanation. Certainly only a minority of the planes appeared in combat. Possibly the remainder were lost in transit from factory to combat bases, destroyed on the fields, or grounded because of a shortage of gasoline or pilots. Conceivably some are part of an inflation of German production figures. The answer is not clear."