Add vscode extensions you want to use below `vscodeExtensions = ...`. You can search for extension names [here](https://search.nixos.org/packages?channel=22.05&from=0&size=50&sort=relevance&type=packages&query=vscode-extensions+extensionYouAreSearchingFor). You may not be able to install extensions through the UI in vscode, I'd recommend always adding them to the flake. If you are inside a `nix develop` shell, run `exit` and `nix develop path-to-your-dev-flake-folder` to be able to run vscode with the newly added extensions.
If your extension is not available on nix, you can add them [from the vscode marketplaces as well](https://stackoverflow.com/a/54812021/4200103).
### for those with little to no Nix experience
Instead of running `code` in the last step you can use the `--extensions-dir` flag to allow you to install extensions using the vscode GUI.