How do the JSONToXML and XMLToJSON policies work in Apigee?

<!-- Client sends JSON; SOAP backend expects XML -->

<!-- Step 1: Convert incoming JSON request to XML -->
<JSONToXML name="JSON2XML.Request">
  <Options>
    <RootElement>root</RootElement>
    <NullValue>NULL</NullValue>
  </Options>
  <OutputVariable>request</OutputVariable>
  <Source>request</Source>
</JSONToXML>

<!-- Step 2: Route to SOAP backend (via TargetEndpoint) -->

<!-- Step 3: Convert XML response from SOAP backend to JSON -->
<XMLToJSON name="XML2JSON.Response">
  <Options>
    <RecognizeNull>true</RecognizeNull>
    <NamespaceSeparator>_</NamespaceSeparator>
    <TextAlwaysAsProperty>false</TextAlwaysAsProperty>
  </Options>
  <OutputVariable>response</OutputVariable>
  <Source>response</Source>
</XMLToJSON>

<!-- Flow placement:
  JSONToXML  -> TargetEndpoint PreFlow Request (transform before backend)
  XMLToJSON  -> TargetEndpoint PostFlow Response (transform after backend)

  Full bridge pattern:
  Client (JSON) -> [JSONToXML] -> Backend (XML/SOAP)
  Client (JSON) <- [XMLToJSON] <- Backend (XML/SOAP)
-->