Reads and writes documents in a customer’s SharePoint site, the place most organisations already keep the files an application needs, rather than a bucket nobody governs. Uses a Graph app registration scoped with Sites.Selected and granted access to one site, so your application can reach that library and nothing else in the tenant. Its variables carry the SHAREPOINT_ prefix rather than GRAPH_, because this is a different registration from the mail one, and an application commonly holds both.
An admin enters these once, and the platform injects them as environment variables at deploy time. Your code reads them. It never sees a login screen or an SDK for this.
SHAREPOINT_TENANT_IDSHAREPOINT_CLIENT_IDSHAREPOINT_CLIENT_SECRETSHAREPOINT_SITE_ID