To integrate with several external systems in Salesforce. According to the Salesforce documentation(1), the answer is:

Platform event
A platform event is a type of event message that your apps can send and receive to communicate changes and respond to them without writing complex logic. Platform events follow a publish/subscribe model, which means that a publisher app can send an event message to a channel, and one or more subscriber apps can listen and react to the event message.

A platform event is suitable for your scenario because:

- It can initiate the process when a record is created in Salesforce by using a record-triggered flow or a trigger to publish an event message.
- It does not require Salesforce to wait for a response from the external systems before continuing, as the event message is sent asynchronously and does not block the execution of the publisher app.
- It can integrate with several external systems by using various tools and protocols, such as Apex, Lightning web components, CometD, or Enterprise Messaging Platform.

The other options are not optimal because:

- An outbound message is a way of sending information from Salesforce to an external system using SOAP protocol. An outbound message requires Salesforce to wait for an acknowledgment from the external system before continuing, which can cause performance issues or errors if the external system is unavailable or slow.

- A PushTopic event is a type of event message that notifies subscribers of changes to Salesforce records that match a SOQL query. A PushTopic event is not suitable for your scenario because it does not allow you to send custom data or logic to the external systems, but only the record data that matches the query.

- A trigger with callout is a way of invoking an external web service from Salesforce using HTTP methods. A trigger with callout requires Salesforce to wait for a response from the external web service before continuing, which can cause governor limit errors o...