After installation:
If you manage 10+ offline workstations, the GUI method is too slow. Enterprises should use the Altium Repository Mirroring tool. This creates a complete local snapshot of Altium’s extension server. altium extension offline install
The UI heavily discourages offline use. The Extensions and Updates homepage is designed to show new releases and updates from the cloud. When working offline, this page becomes a graveyard of broken links and "Connection Failed" icons. After installation: