As of GraphQL API version 2024-10, we are adding new inventory capabilities in the productSet mutation, in the form of a new field type ProductSetInventoryInput, which allows setting available or on_hand quantities for new and existing product variants on locations specified.
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/new-inventory-input-fields-in-productset-mutation-in-version-2024-10