Show an element of a JSON with an ID

I do not see the complication. If as you say, the data is always at the same level, just:

const data = {
  "json": [
    {
      "list_ip": [
        {
          "1": "190.15.20.18",
          "2": "10.10.10.100",
          "3": "10.20.30.40"
        }
      ],
      "cosas": [
        {
          "elementos": [
             {
               "ip_id": 1
             },
             {
               "ip_id": 3
             }
          ]
        }
      ]
    }
  ]
};


function mapIPs() {
  const ips = data.json[0].list_ip[0];
  const els = data.json[0].cosas[0].elementos;
  const associations = {};
  
  els.forEach(el => {
    for(let [key, val] of Object.entries(el)) {
      if (ips[val]) {
        associations[val] = ips[val];
      }
    }
  });
  
  return associations;
}

console.info(mapIPs());

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I leave this example working, since the structure of that JSON is not very useful to relate. In case that the ip_list and the elements were not in position 0 you must do another algorithm to find those variables and store them in the corresponding variables.

    var jsonEstilo1=[
    {
        "list_ip": [
            {
                "1": "190.15.20.18",
                "2": "10.10.10.100",
                "3": "10.20.30.40"
            }
        ],
        "cosas": [
            {
                "elementos": [
                    {
                        "ip_id": 1
                    },
                    {
                        "ip_id": 3
                    }
                ]
            }
        ]
    }
];

    function searchlistIpEstilo1(id){
        //----- La lista de ip asumiento que siempre estará en la posicion 0
        var iplist=jsonEstilo1[0].list_ip[0];
        if(id in iplist){
            return iplist[id];
        }else{
            return null;
        }
    }
    //----- La lista de elmentos asumiento que siempre estará en la posicion 0
      var elementos=jsonEstilo1[0].cosas[0].elementos;
    for(var i=0;i<elementos.length;i++){
        var result=searchlistIpEstilo1(elementos[i].ip_id);
        if(result!=null){
            console.log(result);
        }
}

So I solved it:

        var ips;    
        var ip_ids;
        $.each( data.json, function(index_json, valores ) {
          ips = valores.list_ip
          $.each(valores.cosas.elementos, function(index_elementos, valores_elementos){
            ip_ids = valores_elementos.ip_id;
            $.each(ips, function(index_ip, valor_ips){
                    ip_el = valor_ips[ip_ids];
            });
          console.log(ip_el);
          });
        });

Here is an example of how to extract the ip according to the value of ip_id :

obj = {
 "json": [{
  "list_ip": [{
   "1": "190.15.20.18",
   "2": "10.10.10.100",
   "3": "10.20.30.40"
  }],
  "cosas": [{
   "elementos": [{
    "ip_id": 1
   },{
    "ip_id": 3
   }]
  }]
 }]
}
display(obj,"","ip_id");

function display(xobj,sp,search) {
 for (n in xobj) {
  if (typeof xobj[n] == 'object') {
   display(xobj[n],n+".");
  }else{
   if (n=="ip_id")
    html.innerHTML+="<p>"+sp+n+"="+xobj[n]+" ("+obj.json[0].list_ip[0][xobj[n]]+")";
  }
 }
}

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