Hi,
In SharePoint 2013, visual studio workflows run in workflow manager.
“
Microsoft has taken a very different approach to workflows in SharePoint 2013 than in previous versions. SharePoint workflows are now based on Windows Workflow Foundation 4, and their execution is driven by a
new component called Workflow Manager, which runs externally to SharePoint.
”
The new workflow architectural for your reference:

SharePoint 2010 workflow API was manifested only in the server-side object model, but SharePoint 2013, Microsoft extended the CSOM to include a robust API for the new workflow architecture.
And the workflow manager provides new features like enable no-code web service calls from within a workflow, new workflow building blocks such as Stage, Loop, and App Step, more information for your reference:
https://technet.microsoft.com/en-us/library/jj219638.aspx
The improvements of SharePoint 2013 workflow for your reference:
http://blog.tahoepartners.com/index.php/sharepoint-2013-workflows-the-top-5-improvements/
Best Regards,
Dean Wang
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