The allies defeated Germany by fielding more men, tanks, guns, ships, aircraft and supplies than the Germans. They did also beat the Germans at their own game: Mechanized warfare.
The Normandy Invasion in the West and Operation Bagration in the East were key moments and the Allies pushed the front back to the German borders. Massive offensives crushed the weakening power of the German Armed forces. The German's strength was ground down over time.
None of the German weapons research came soon enough to significantly blunt the Allies attacks. In particular their nuclear research did not pan out. The lack of many Strategic resources like oil hindered operations.
The German Armies were still significantly holding back the Allied Armies as late as April 1945. German training and weapons were very advanced for their time.
Allied Strategic Air attacks also contributed and Allied cryptography (Enigma/Ultra) was a major asset.
The Allied agreement not to negotiate separately with Germany probably extended the War and made the war more costly in lives.
Germany also lacked friends big and small at the end of the war by their brutal treatment of almost everyone. Whole countries like Romania switched sides when the going got tough.
Hitler made plans to poison water supplies and other scorched earth type actions in Germany. Common sense prevailed and most of these plans were not carried out.
https://www.amazon.ca/Dirty-Little-Secrets-World-War/dp/0688122884
https://www.amazon.com/History-Second-World-B-H-Liddell/dp/156852627X
