
der Aufbau
structure
The structure, arrangement, or organization of something.
Der logische Aufbau des Arguments hat mich überzeugt.
The logical structure of the argument convinced me.
structure / assembly
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structure
The structure, arrangement, or organization of something.
Der logische Aufbau des Arguments hat mich überzeugt.
The logical structure of the argument convinced me.

assembly / setup
The process of erecting, assembling, or putting something together.
Der Aufbau des neuen Schranks dauerte mehrere Stunden.
The assembly of the new cabinet took several hours.

rebuilding / development
The process of rebuilding or development after destruction or a crisis.
Der Aufbau der Wirtschaft nach dem Krieg war eine große Herausforderung.
The rebuilding of the economy after the war was a major challenge.

body / superstructure
A component mounted on the base part or chassis of a vehicle or machine.
Der LKW bekommt einen speziellen Aufbau für den Transport von Kühlwaren.
The truck is getting a special body for transporting refrigerated goods.
'Der Aufbau' (structure/assembly) is the noun form of the separable verb 'aufbauen' (to build up/set up). It describes both the process of 'Aufbauen' (building up/setting up) and the result, meaning the finished structure.
Der Aufbau des Immunsystems dauert bei Kindern mehrere Jahre.
The development of the immune system takes several years in children.
'Aufbau' (structure/assembly) can describe an abstract structure (e.g., of a text) or a concrete process (e.g., assembling furniture). The context makes the difference clear.
Der Aufbau deiner E-Mail war sehr unklar.
The structure of your email was very unclear.
When 'Aufbau' (structure) means a structure, it is often combined with abstract things. Typical phrases are 'der Aufbau eines Romans' (the structure of a novel), 'der logische Aufbau eines Arguments' (the logical structure of an argument), or 'der Aufbau einer Zelle' (the structure of a cell).
Für die Prüfung müssen wir den Aufbau des menschlichen Herzens lernen.
For the exam, we have to learn the structure of the human heart.
In the meaning of 'Montage' (assembly) or 'Errichtung' (construction/erection), 'Aufbau' refers to the physical putting together of parts. It is often about furniture, tents, or technical devices.
Die Anleitung für den Aufbau des Bettes war sehr kompliziert.
The instructions for assembling the bed were very complicated.
This meaning often describes large, social processes. It is about restoration after a crisis, e.g., 'der Aufbau einer Region nach einer Naturkatastrophe' (the rebuilding of a region after a natural disaster).
Nach seiner langen Krankheit konzentrierte er sich auf den Aufbau seiner Fitness.
After his long illness, he focused on rebuilding his fitness.
organization
Organisation and Aufbau can both describe the internal structure of an authority. Aufbau focuses mainly on the arrangement of the parts and their hierarchy, while Organisation also includes how they function together. When the focus is on departments and levels, Aufbau works; when the focus is on their cooperation, Organisation is the better choice.

Der Aufbau der Behörde mit drei Abteilungen ist klar gegliedert.
The structure of the authority, with three departments, is clearly organized.
Die Organisation der Behörde ermöglicht eine gute Zusammenarbeit der Abteilungen.
The organization of the authority enables good cooperation between the departments.