M2 system use  for read and execute (scriptSelector = 'script[type="text/x-magento-init"]' and dataAttr = 'data-mage-init' )  many js file like vendor/magento/magento2-base/lib/web/mage/apply/main.js  and scripts.js and vendor/magento/magento2-base/lib/web/mage /mage.js
you can find vendor/magento/magento2-base/lib/web/mage 
in normal case  you can declare as virtual :

 
        <script type="text/x-magento-init"> 
        { 
            "[data-role=tocart-form], .form.map.checkout": { 
                "catalogAddToCart": {} 
            } 
        }
       </script>
 

or simple :

 
data-mage-init='{"catalogAddToCart": {}}'
 

vendor/magento/magento2-base/lib/web/mage/apply/scripts.js  is responsible for parses 'script' tags with a custom type attribute and moves it's data
     * to a 'data-mage-init' attribute of an elemennt found by provided selector.
     * Note: All found script nodes will be removed from DOM

and mage.js take a hand for  plugin mage, initialize components on elements  :

 
    $.fn.mage = function (name, config) {
        config = config || {};
        this.each(function (index, el) {
            mage.applyFor(el, config, name);
        });
      return this;
    };
 

Here mage.js iterate all element in dom was declared (scriptSelector = 'script[type="text/x-magento-init"]' and dataAttr = 'data-mage-init' ) applied in it . For that it use mage.applyFor(el, config, name) ,

the name in our example is :  catalogAddToCart Components path  in declared inside requirejs-config.js 
config : in our case we passed any configuration you can pass template variable in general or sub child component .

 in bottom we Init components inside of dynamically updated elements:

 
    /**
     * Init components inside of dynamically updated elements
     */
    $('body').on('contentUpdated', function () {
        if (mage) {
            mage.apply();
        }
    });
 

we have here 2 functions apply and applyFor is inside vendor/magento/magento2-base/lib/web/mage/apply/main.js  ,ok applyFor is equal init function 
 

 
  /**
     * Initializes components assigned to a specified element via data-* attribute.
     *
     * @param {HTMLElement} el - Element to initialize components with.
     * @param {Object|String} config - Initial components' config.
     * @param {String} component - Components' path.
     */
    function init(el, config, component) {
        require([component], function (fn) {
            if (typeof fn === 'object') {
                fn = fn[component].bind(fn);
            }
            if (_.isFunction(fn)) {
                fn(config, el);
            } else if ($(el)[component]) {
                $(el)[component](config);
            }
        });
    }
 

and apply Initializes components assigned to HTML elements via [data-mage-init]. 

*      @example Sample 'data-mage-init' declaration.
*      data-mage-init='{"path/to/component": {"foo": "bar"}}'

it call the function init many time :

 
        apply: function () {
            var virtuals = processScripts(),
                nodes = document.querySelectorAll(nodeSelector);
            _.toArray(nodes)
                .map(getData)
                .concat(virtuals)
                .forEach(function (itemContainer) {
                    var element = itemContainer.el;
                     _.each(itemContainer.data, function (obj, key) {
                            if (obj.mixins) {
                                require(obj.mixins, function () {
                                    for (var i = 0, len = arguments.length; i < len; i++) {
                                        $.extend(true, itemContainer.data[key], arguments[i](itemContainer.data[key], element));
                                    }
                                   delete obj.mixins;
                                    init.call(null, element, obj, key);
                                });
                            } else {
                                init.call(null, element, obj, key);
                            }
                        }
                    );
              });
        },
        applyFor: init
    };
 

that is general view of component ...