I hope It will be helpful to you.

1. you open vscode and go to the extension menu.
2. you choose what you want download vsix.
3. but It should be not installed on your vscode. If you install it, you should uninstall VSIX.
4. You click right button mouse, then you can see the download VSIX
5. and you choose platform and download it.

Belows are link and text.

https://code.visualstudio.com/docs/editor/extension-marketplace#_can-i-download-an-extension-directly-from-the-marketplace

Can I download an extension directly from the Marketplace? Some users prefer to download an extension once from the Marketplace and then install it to multiple VS Code instances from a local share. This is useful when there are connectivity concerns or if your development team wants to use a fixed set of extensions.

To download an extension, search for it in the Extensions view, right-click on an extension from the results, and select Download VSIX.