The use of above mentioed manager classes is to call some tools classes for low level operations(operation on repository add, update etc) and we call pipeline method. i.e. To update order we call CartModifierFormHandler.handleSetOrder->CartModifierFormHandler.updateOrder->OrderManager.updateOrder->PipelineManager.runProcess.
High level operation is nothing but any operation that we want to perform before performing operation in repository by calling tools class(low level operation). For example to update order, In OrderManager class we create some required instance of different components in a sequence (mutrep, transaction blocks , calling pipeline method and various logic ect) and tools class basically have set of methods, each performing particular operation in repository, called low level operation. Calling them in a right sequence is done by ordermanager class. All this operation performed by ordermanager call is known as high level operation.