In order to bring the total weight down to 643 grams, Fujitsu had to reduce the number of components, replace the metallic upper cover with carbon fiber and redesign some of the ports and internal circuitry. The UH-X / E3 models are powered by the Intel Core i7-1165G7 CPU and includes a small 25 Wh battery, but Fujitsu also offers the slightly heavier UH90 / E3 models with 54 Wh batteries and the UH75 / E3 models powered by the AMD Ryzen 7 4700U APUs that weigh just under 1Kg.
https://www.notebookcheck.net/Fujitsu-announces-the-world-s-lightest-13-3-inch-Lifebook-laptops-weighing-only-634-g-Intel-Tiger-Lake-and-AMD-Renoir-processors-in-tow.498796.0.html