Getting values out of a JSON list of objects with Apex code.

json-deserialization.java

// JSON DESERIALIZATION (API RESPONSE OR OTHER)
// --------------------

// Deserialize and cast JSON string into single object
Map<String,Object> jsonObjectMap = (Map<String,Object>)JSON.deserializeUntyped('{"id":"001","email":"email@email.com"}');

String externalId = String.valueOf(jsonObjectMap.get('id'));
    
// --------------------

// Deserialize and cast JSON string into a list of objects
List<Object> jsonObjectsList = (List<Object>)JSON.deserializeUntyped('[{"id":"001","email":"email@email.com"},{"id":"002","email":"email2@email.com"}]');

for(Object jsonObjectString : jsonObjectsList){
    
    // Cast to JSON string to Apex map
    Map<String,Object> jsonObjectMap = (Map<String,Object>)jsonObjectString;
    
    System.debug(String.valueOf(jsonObjectMap.get('id')));
}