The API Connector and Plugin Editor’s API tab allow you to setup access to third-party APIs.
See these links for more details: https://bubble.is/reference#Plugins and https://manual.bubble.is/building-plugins/adding-api-connections.html
Once those are setup you can use them as Data or Action types in your Bubble app.
API Workflows in Bubble are server-side workflows. They do not require User interaction and allow the User to continue using your app without seeing a loading bar on the screen – basically the process has been handed off to the server for processing. They are not the same as setting up a third-party API in the API Connector or Plugin Editor. More details here: https://bubble.is/reference#Actions.TriggerCustomEventFromReusable.workflow_thing
API workflows (this could really be renamed “Server-side workflows”) may, or may not, use an API you setup in the API connecter or Plugin editor.
Here is a simple API workflow that does not use a third-party API. It is only used to create a record in the background, which doesn’t affect the User while they are browsing the site, since all processing happens on the server.
Here is another example that shows an API workflow using a third-party API that I setup in the Plugin Editor.