The United States was helping Great Britain with the Lend Lease program, before the British offered any technology in Reverse Lend Lease. They did so because American industry could mass produce these devices far better than the British could. Important items that were reverse lend leased to the United States include:
The Rolls Royce Merlin aircraft engine. The Packard Motor company was licensed to build these V-12 motors. They were better than the V-1710 model V-12 engine designed by Allison, in the USA. Packard Merlins were used on the North American P-51 Mustang and resulted in a major improvement in performance, compared to the early Allison engine P-51s. Packard Merlins were also used on late model Curtis P-40s, but the jump in performance on that plane was not as dramatic. Packard also supplied Merlins to the Canadian factories that were building the Avro Lancaster bombers, for the RAF.
The Cavity Magnitron was another notable piece of technology transferred to the US. It was an amplifier that allowed radar sets to emit centrimetric radar waves. This greatly improved radar sets, compared to the earlier sets that emitted long wave radar signals. Centrimetric radars were small enough to fit in an airplane and could detect objects as small as a submarine periscope. The British electrical industry could hand build 24 centrimetric radar sets each month, but the industrial engineers in the US, could churn out 150 sets per week. It helped to turn the tide in the Battle of the Atlantic in 1943. There was a whole array of anti-submarine technology, like the Squid and Hedgehog that were invented in the UK and shared with the US Navy.
The British invented the escort aircraft carrier, but only built 21 of these baby flattops during the war. US shipyards built 113 of them. The British also designed the Landing Ship Tank, other smaller amphibious assault ships like the Landing Craft Tank (LCT) and a bare bone merchant ship (the Liberty Ship) and relied on American industry to build them in vast quantities. One noted American industrialist, Henry J Kaiser, would make the members of the British Purchasing Commission cringe, because he refused to use proper nautical terms, when describing how his ship yards could literally "Build them faster than they can sink them" but they did not complain when he built Liberty ships by the thousands.
The British also shared their early turbo jet aircraft engine technology. However, the first American jet war plane, the Bell XP-59 was a dud. The P-51 and several other American propeller planes were faster than the XP-59 and it never entered combat. Late war British turbo jet engine designs were licensed to General Electric and they were used on the much better Lockheed P-80. However, the war was over before the P-80s were deployed to the combat zone.

Part II
Why China lost its lead in technology is a problem that vexes many historians. This web-site will get you started on the reasons. The TL/DR answer is the Ming Dynasty was rather conservative and it rested on its laurels. It thought its products were better than European products and was willing to accept European silver in return for products that the Europeans wanted. like silk, tea and ceramics.
http://anexerciseinfutility.blogspot.com/2013/03/why-did-china-fall-behind.html
By 1841, during the Opium Wars, the Royal Navy was so far ahead of the Chinese in Naval technology, it did not steal anything from China. The British had wooden ships with lanteen sails, stern post rudders and powerful cannons that could blow any Chinese junk out of the water. The Royal Navy had honed its fighting skills for some two hundred and fifty years, fighting the Spanish, Dutch and French in a dozen different wars. By 1841, the Royal Navy was the strongest navy in the world.
Sources: "Winged Victory: The US Army Air Force in World War II" by Geoffrey Perret
"Engage the Enemy more Closely: The Royal Navy in the Second World War" by Correlli Barnett
"The Rise and Fall of the Great Powers" by Paul Kennedy
"The Rise and Fall of British Naval Mastery" by Paul Kennedy