To update the accounts from the external system, the answer is: Bulk API

Bulk API is a way of importing or exporting large amounts of data in Salesforce using RESTful principles. Bulk API allows you to process data in batches, which can contain up to 10,000 records or 10 MB per file. Bulk API is asynchronous, which means that you can submit a job and check its status later, without waiting for it to finish. Bulk API is suitable for your scenario because:

- It can handle millions of accounts that may be updated in Salesforce every quarter.
- It can perform high-performance updates of existing records using the upsert operation.
- It can leverage the existing security features of Salesforce, such as authentication and encryption.
- It can integrate with any external system that supports HTTP requests and JSON or XML formats.

The other options are not optimal because:

- Apex REST Web Service is a way of exposing custom logic and data in Salesforce as RESTful APIs. Apex REST Web Service is not suitable for your scenario because it does not support bulk operations or asynchronous processing, and it requires writing custom Apex code.
- Composite REST API is a way of combining multiple requests into a single call in Salesforce. Composite REST API is not suitable for your scenario because it has a limit of 25 subrequests per call, and it does not support bulk operations or asynchronous processing3.
- SOAP API is a way of accessing data and functionality in Salesforce using SOAP protocol. SOAP API is not suitable for your scenario because it has a limit of 200 records per call, and it does not support asynchronous processing or JSON format.

https://trailhead.salesforce.com/content/learn/modules/api_basics/api_basics_overview...