where is AEM getting the priority from for the workflows inbox

The logic to fetch or create data for TouchUI inbox is at the Java side, below is the basic flow:

~ A request to '/aem/inbox' resolves to path '/libs/cq/inbox/content/inbox'.

~ This renders data per [1] which creates the html page per [2].

~ At the same time [1] triggers a get request to fetch the data for the inbox page [3] which is handled by a servlet [4] and used by [2] to create the complete inbox page.

~ Further on logic goes to [5] and [6] to fetch the data per the logic.

[0]: /libs/cq/inbox/content/inbox

[1]: /libs/cq/inbox/content/inbox/jcr:content/views/list/datasource

[2]: /libs/cq/inbox/gui/components/inbox/inboxitem/list/list.html

[3]: cq/inbox/gui/components/inbox/datasource/itemsdatasource

[4]: com.adobe.cq.inbox.impl.servlet.ItemsDataSourceServlet.java

[5]: com.adobe.granite.workflow.core.WorkflowSessionImpl.java

[6]: com.adobe.granite.workflow.core.jcr.WorkItemManager.java

For workflows it is medium by default. You can change it though in process step using WorkItem API.

https://helpx.adobe.com/experience-manager/6-3/sites/developing/using/reference-materials/javadoc/com/adobe/granite/workflow/exec/WorkItem.html#setPriority(com.adobe.granite.workflow.exec.InboxItem.Priority)