Konrad Zuses's Z4 relay machine (1945) is the world's oldest preserved computer. It was used by the ETH Zurich from 1950 to 1955 and is now located in the Deutsches Museum, Munich.
The user manual was lost for a long time, it now came to light:
Herbert Bruderer: Milestones in Analog and Digital Computing, Springer Nature Switzerland
AG, Cham, 3rd edition 2020, 2 volumes, 2050 pages, 715 illustrations,
151 tables,