Write a SOQL query to fetch all Users who are assigned a specific Permission Set (PermissionSetAssignment where PermissionSet.Name = 'Sales_Ops').

Query PermissionSetAssignment and filter on the permission set name, selecting the assigned user details through the Assignee relationship.

SELECT Assignee.Id, Assignee.Name, Assignee.Username
FROM PermissionSetAssignment
WHERE PermissionSet.Name = 'Sales_Ops'

The API name goes in PermissionSet.Name, not the label.