The O.P. of thread that I linked lists all of the past and current complete stock system builds, their bootloaders and their stock recoveries, as well as the various radios. All categorized by build number.
All of the files listed including the stock system builds, bootloaders, stock recoveries and radios are flashable zips; which are all flashed in TWRP.
It is my understanding that using these are the simplest way to update to 5.1.1.