What are the differences between conversation, intents, intent_script, and responses?

I'm confused by the different elements of HA's voice assistant sentences.

1. What's the difference between a conversation and an intent_script? Per HA's custom sentence example, a conversation has an intents sub-element, and an intent_script doesn't. Does a conversation's intent merely declare the element that will respond to the sentence, while an intent_script is purely the response (i.e., does an intents point to an intent_script)?

2. HA then explains that while the example above defined the conversation and intent_script in configuration.yaml, you can also define intents in config/custom_sentences/. Should you use both of these methods simultaneously or will it cause conflict or degrade performance? I wouldn't think you should define the same sentence in both places, but the data structure for their 2 examples are different - is 1 better than the other?

In configuration.yaml:

conversation:
  intents:
    YearOfVoice:
      - "how is the year of voice going"

In config/custom_sentences/en:

intents:
  SetVolume:
    data:
      - sentences:
          - "(set|change) {media_player} volume to {volume} [percent]"
          - "(set|change) [the] volume for {media_player} to {volume} [percent]"

3. Then they say responses for existing intents can be customized as well in config/custom_sentences/. What's the difference between a response and an intent_script? It seems like intent_script can only be defined in configuration.yaml and responses can only be defined in config/custom_sentences/` - is that right?

Thanks for any clarification you can share.