If you only need the cumulative update for 21H2 (not the full OS):

This method downloads a standalone installer that works without an active internet connection after download.

This is the standard utility to create a bootable USB or download an ISO file. While the current tool on the Microsoft Download page typically defaults to version 22H2, older versions like 21H2 can sometimes be accessed through archived links or third-party open-source scripts.

Visit the official Microsoft Software Download page (formerly Media Creation Tool).

Copy the file to the offline PC and double-click to install.

Download the .msu file matching your system architecture.

: This package acts as a master switch, requiring only a single restart.