How to check which service quotas are used by a Web App

Question

Currently I am using Google Sheets as a database in my app. I am aware of the usage quotas, but I am confused about which services I am using and what the limitations are. My app just passes values to Google Sheets. I would like to get some answers for the following questions:

1. Triggers total runtime = 90min/day
   Is this the total time that the script would run in a day?

2. URL fetch calls = 20000/day
   Does it mean I can call script url 20000 per day from my app?

3. Script runtime = 6min/execution
   Please explain what this quota means

Web App code:

private void setItems() {

    StringRequest stringRequest = new StringRequest(Request.Method.POST, script,
                        new Response.Listener<String>() {
                            @Override
                            public void onResponse(String response) {

                        },
                        new Response.ErrorListener() {
                            @Override
                            public void onErrorResponse(VolleyError error) {

                            }
                        }
                ) {
                    @Override
                    protected Map<String, String> getParams() {
                        Map<String, String> parmas = new HashMap<>();

                        //here we pass params
                        parmas.put("action","addItem");
                        parmas.put("teamname",teamname);
                        parmas.put("p1",finalp1);
                        parmas.put("p2",finalp2);
                        parmas.put("p3",finalp3);
                        parmas.put("p4",finalp4);
                        parmas.put("p5",finalp5);

                        return parmas;
                    }
                };

                    int socketTimeOut = 50000;// u can change this .. here it is 50 seconds

                    RetryPolicy retryPolicy = new DefaultRetryPolicy(socketTimeOut, 0, DefaultRetryPolicy.DEFAULT_BACKOFF_MULT);
                    stringRequest.setRetryPolicy(retryPolicy);

                    RequestQueue queue = Volley.newRequestQueue(JoinContest.this);

                    queue.add(stringRequest);

Server-side code:

var ss = SpreadsheetApp.openByUrl("MY URL");

var sheet = ss.getSheetByName('Main');

function doPost(e){
var action = e.parameter.action;

if(action == 'addItem'){
  return addItem(e);

}

}

function addItem(e){

var teamname =  e.parameter.teamname;

var position  =  sheet.getLastRow(); // Item1

var p1 = e.parameter.p1;

var p2 = e.parameter.p2;

var p3 = e.parameter.p3;

var p4 = e.parameter.p4;

var p5 = e.parameter.p5;  
sheet.appendRow([position,teamname,p1,p2,p3,p4,p5]);

return ContentService.createTextOutput("Success").setMimeType(ContentService.MimeType.TEXT);

}