You can include associated entities by using expand, e.g. order with orderlines. See: https://www.odata.org/getting-started/basic-tutorial/#expand

The oData contract, $metadata, describes an entire domain model, including associations between entities. You can use expand to join entities over these associations.

You can request multiple queries in one http operation using batch requests: https://www.odata.org/getting-started/advanced-tutorial/#bat...