You can create a test case in Azure Test Plans directly from a Work Item (e.g. a Product Backlog Item or Bug) and also from a Test Plan or Test Suite.
Creating a test case from a Work Item
You can create a new test case from any list of work items (e.g. a Sprint Backlog).
Creating a test case from a Test Plan or Test Suite
You can also create a new test case from a Test Plan or Test Suite.