I think you will need internet for SharePoint framework development. Especially for initial setup.
Then you will need internet for downloading npm packages for first time (Or maybe you can achieve this using workarounds like this and this). Then you can copy node modules folder with dependencies to your other project folders. Alternatively, you can also use Yarn-offline-mirror method (Reference).
Once you are done with initial setup then you won't need internet while you are developing your SPFx components.
But you will again need internet for debugging on SharePoint workbench and deploying solutions to SharePoint environment.
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