You have to use this function from PHAsset object:
- (PHContentEditingInputRequestID)requestContentEditingInputWithOptions:(nullable PHContentEditingInputRequestOptions *)options completionHandler:(void (^)(PHContentEditingInput *__nullable contentEditingInput, NSDictionary *info))completionHandler;
Then you can retrieve URL this way:
NSURL *url = contentEditingInput.fullSizeImageURL;
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