Explanation
internal apps can use processes, flows, or Apex to publish platform event messages from a Salesforce app.
Process Builder is a tool that lets you automate business processes by creating processes. You can use Process Builder to publish platform event messages when a record changes or when a platform event occurs.
external apps can use Salesforce APIs to publish platform event messages. You can use any Salesforce API to create platform events, such as SOAP API, REST API, or Bulk API. The Streaming API is a Salesforce API that lets you push a stream of notifications from Salesforce to external apps. You can use the Streaming API to publish platform event messages by creating records of your event type.
Therefore, using Process Builder and Streaming API are two mechanisms to publish platform event messages after a platform event is defined in a Salesforce org.
https://developer.salesforce.com/docs/atlas.en-us.platform_events.meta/platform_events/platform_events_publis