The code is based on the Hello World sample extension that demonstrates how to set up and use a SolidJS + Webview UI Toolkit webview extension.
Once you've pulled the repo you need to install dependencies.
# Navigate to project folder
# Install dependencies for both the extension and webview UI source code
npm run install:all
# Build webview UI source code
npm run build:webview
# Open project in VS Code
code .
Note that when you change the webview you must rebuild it using the NPM script (bottom of explorer window) or the command above.