1. dom selector

In dojo/dom.js,dojo offers dojo.byId() to get the dom element node.

dom.byId = function(id,doc){
			// inline'd type check.
			// be sure to return null per documentation,to match IE branch.
			return ((typeof id == "string") ? (doc || win.doc).getElementById(id) : id) || null; // DOMNode
		};

dojo/query.js provides the query API to get the NodeList.

var query = queryForEngine(defaultEngine,NodeList);

function queryForEngine(engine,NodeList){
		var query = function(/*String*/ query,/*String|DOMNode?*/ root){
			// summary:
			//		Returns nodes which match the given CSS selector,searching the
			//		entire document by default but optionally taking a node to scope
			//		the search by. Returns an instance of NodeList.
			if(typeof root == "string"){
				root = dom.byId(root);
				if(!root){
					return new NodeList([]);
				}
			}
			var results = typeof query == "string" ? engine(query,root) : query ? query.orphan ? query : [query] : [];
			if(results.orphan){
				// already wrapped
				return results;
			}
			return new NodeList(results);
		};
		query.matches = engine.match || function(node,selector,root){
			// summary:
			//		Test to see if a node matches a selector
			return query.filter([node],root).length > 0;
		};
		// the engine provides a filtering function,use it to for matching
		query.filter = engine.filter || function(nodes,root){
			// summary:
			//		Filters an array of nodes. Note that this does not guarantee to return a NodeList,just an array.
			return query(selector,root).filter(function(node){
				return array.indexOf(nodes,node) > -1;
			});
		};
		if(typeof engine != "function"){
			var search = engine.search;
			engine = function(selector,root){
				// Slick does it backwards (or everyone else does it backwards,probably the latter)
				return search(root || document,selector);
			};
		}
		return query;
	}

We can find thebyId() method in the dijit/registry.js

byId: function(/*String|Widget*/ id){
			// summary:
			//		Find a widget by it's id.
			//		If passed a widget then just returns the widget.
			return typeof id == "string" ? hash[id] : id;	// dijit/_WidgetBase
		},