SharePoint 2016: Get access token

I need to invoke intranet SharePoint 2016 REST API's to retrieve items. My options are CSOM (or JSOM) or REST APIs. I plan to use REST API's which needs an access token. I've looked up documentation but nothing seems straightforward. Does anybody have suggestions on how I can get access token w/o asking user for credentials each time.

In addition, Im trying to get list of new documents that have been uploaded between subsequent checks. Is there a way to do this? Think of differential updates vs full.

You can get SharePoint 2016 form digest value by doing a POST Request like this:

   $.ajax 
    ({   
        url: _spPageContextInfo.webAbsoluteUrl + "/_api/contextinfo",   
        type: "POST",   
        async: false,   
        headers: { "accept": "application/json;odata=verbose" },   
        success: function(data){   
            var FormDigestValue = data.d.GetContextWebInformation.FormDigestValue; 
            console.log(FormDigestValue);            
        },   
        error: function (xhr, status, error)
        {
              console.log("Failed");
        }  
    });

Then use this digest value in ajax request headers when connecting to SharePoint using Rest API:

Reference:

Get Form Digest Value in SharePoint using REST API

SharePoint – How to Authenticate SharePoint REST APIs with REST Client