What is the purpose of some Elasticsearch document fields to have @ and _ at the beginning and some duplicates

Question

Because some fields of document of Elasticsearch have at its beginning the characters "@" and "_" and are duplicated ex:

@ at first: @tags, @type
_ at first: _score, _type

Some fields have two versions one with "@" and one with "_" (with the same value):

@version e _version

Some fields have two versions one with "@" and one without:

fields(dentro do root) e @fields(dentro do _source)

Some fields are duplicated (and with the same value):

@timestamp(dentro do fields) e @timestamp(dentro do _source)

Below a document real of Elasticsearch with these fields:

{
  "_index": "logstash-2021.02.25",
  "_type": "_doc",
  "_id": "q9_C1ncBR7kZ5_B9FyW_",
  "_version": 1,
  "_score": null,
  "_source": {
    "arquivo": "C:\\DEV_HOME\\GeradorDePlanilha.cs",
    "level": "INFO",
    "@tags": [],
    "@type": "amqp",
    "date": "2021-02-24 22:16:20.0910",
    "@version": "1",
    "type": "App.Log",
    "metodo": "GerarPlanilhaComDadosGerenciaisBasicosAsync",
    "@timestamp": "2021-02-25T01:16:20.091Z",
    "origin": "App.Api.Worker",
    "@fields": {},
    "@source": "nlog://DESKTOP-F8BDSSI/API.Gerencia.GeradorDePlanilha",
    "logger": "App.Log.Gerencia.GeradorDePlanilha",
    "@message": "Método: GerarPlanilhaComDadosGerenciaisBasicosAsync Arquivo: C:\\DEV_HOME\\GeradorDePlanilha.cs Linha: 33",
    "machineName": "DESKTOP-F8BDSSI"
  },
  "fields": {
    "metodoCompleto": [
      "GerarPlanilhaComDadosGerenciaisBasicosAsync - C:\\DEV_HOME\\GeradorDePlanilha.cs"
    ],
    "@timestamp": [
      "2021-02-25T01:16:20.091Z"
    ]
  },
  "sort": [
    1614215780091
  ]
}