first of all make sure you are using Medusa V2 and not V1. then follow the docs to add the relationship using the link module.

1. Create the Module, name it after what the table is e.g., Vendor,
2. Create the model, Service and index.ts in the module folder
3. Create the link in /links
4. Create and Run migrations

to extend functionality make use of subscribers, overriding apis, hooks, etc

most are in the website docs