Search: O(logn): it must traverse down the entire tree to find the element. To be specific the log in this case is log_4, as there are 4 children.
Insert(single point): O(logn): You must traverse the tree place to find the insertion location, then some small constant amount of work to split the points in that quadrant.
Insert(n points): O(nlogn), every point must inserted, leading to nlogn. I hope this is what the other site you read meant be nlogn, otherwise they would be very wrong.
The original paper is called "Quad trees a data structure for retrieval on composite keys" by Finkel and Bentley.