.env.local always overrides the defaults set.
Good to know: .env, .env.development, and .env.production files should be included in your repository as they define defaults. .env*.local should be added to .gitignore, as those files are intended to be ignored. .env.local is where secrets can be stored.
Environment Variables on Vercel
When deploying your Next.js application to Vercel, Environment Variables can be configured in the Project Settings.
All types of Environment Variables should be configured there. Even Environment Variables used in Development – which can be downloaded onto your local device afterwards.
If you've configured Development Environment Variables you can pull them into a .env.local for usage on your local machine using the following command:
Terminal vercel env pull .env.local
Test Environment Variables