Mercur is built as a set of Medusa plugins, ready to be pulled from NPM and installed in your project. If you want to modify or add new features to Mercur, you don’t want to use prebuilt packages, but set up a development version. Step 1: Fork MercurDocumentation Index
Fetch the complete documentation index at: https://docs.mercurjs.com/llms.txt
Use this file to discover all available pages before exploring further.

.env file with your variables.
Step 5: Create database, migrate, and create admin account
Optionally: run
yarn seed to fill database with example data/apps/backend directory!)
yarn build command. Optionally, you can run yarn dev in both the plugin directory and the backend app—all the changes will be built automatically, and the backend will restart.
More on plugins: Medusa Plugin Docs