Two ways that this off-platform job can be established without additional third-party tools are:
Install a minimal set of dev tools on a machine such as the Command Line Interface (CLI) and create appropriate scripts to import files containing the data. The CLI is a tool that allows developers to interact with Salesforce from the command line. The developer can use the CLI to create scripts that use commands such as force:data:bulk:upsert or force:data:tree:import to import data from CSV or JSON files into Salesforce.
Authorize a machine against the Salesforce org's APIs with appropriate security measures and create a script to call the APIs. The APIs are interfaces that allow developers to access data and functionality in Salesforce from external systems. The developer can use the APIs, such as REST API or Bulk API, to create a script that authenticates with Salesforce using OAuth 2.0 or JWT, and then performs operations such asinsert, update, or delete on Order data.
Verified References: https://developer.salesforce.com/docs/atlas.en-us.sfdx_cli_reference.meta
/sfdx_cli_reference/cli_reference_force_data.htm https://developer.salesforce.com/docs/atlas.en-us.api_rest.
meta/api_rest/intro_what_is_rest_api.htm https://developer.salesforce.com/docs/atlas.en-us.api_asynch.meta
/api_asynch/asynch_api_intro.htm

One invoice will be created when using the Create Invoice Apex action in a flow. The Create Invoice Apex action is a custom action that creates an invoice for an order summary. An invoice is a document that requests payment from a customer for an order. An invoice can include one or more fulfillment orders and one or more credit memos. A fulfillment order is a record that represents a part of an order that is fulfilled by a specific location. A credit memo is a record that represents a partial or full refund for an order or a fulfillment order.
An order summary can have multiple fulfillment orders and credit memos associated with it, but only one invoice will be created for the entire order summary. References: Order Management Objects, [Create Invoice Apex Action]

Three steps that the administrator should take to implement this functionality are:
Create an External Data Sourcepointing to the external system's database. An External Data Source is a type of metadata that defines the connection and authentication information for an external system that stores data outside of Salesforce. The administrator can create an External Data Source using the Lightning Connect OData 2.0 or 4.0 adapters, which support read-only access to data in relational databases.
Create an External Object for each table of accounting data in the external system. An External Object is a type of custom object that maps to a table in an external system. The administrator can create an External Object for each table of accounting data that they want to access from Salesforce, and define its fields and relationships accordingly.
Create an Indirect Lookup field to connect to the Account. An Indirect Lookup field is a type of custom field that links a child External Object record to a parent standard or custom object record. The administrator can create an Indirect Lookup field on the External Object that references the Account object, and specify the external column name and value that match the Account ID.
Verified References: https://help.salesforce.com/s/articleView?id=sf.external_object_overview.
htm&type=5https://help.salesforce.com/s/articleView?id=sf.external_object_relationships.htm&type=5