Hi @aravindhrs,
A Send Task is a more specific type of Service task from the semantics perspective. On the implementation level, both have the same behavior [1].
A Receive Task will wait for an BPMN message in the engine. You will need an external service which correlates emails to the processes an triggers messages. [2, 3]
As for the example, the poll pizza orders has to be a service task as it actively checks the mail. For the send Tasks, it does not matter because it’s the same behavior.
Unfortunately I don’t know the answer you with your last question but I hope I could help you anyways 
[1] https://docs.camunda.org/manual/latest/reference/bpmn20/tasks/send-task/
[2] https://docs.camunda.org/manual/7.11/reference/bpmn20/tasks/receive-task/
[3] https://docs.camunda.org/manual/latest/reference/rest/message/post-message/