www.makemkv.com

i was about to follow this guide https://www.dvd-guides.com/guides/blu-r ... ray-to-mkv but what is the difference between remuxing a bluray and using makemkv to save it as .mkv? i was reading that if i want to do something with like audio tracks etc, i can remux it. so my other question is, should i save the bluray as .mkv or should i save it as a disk backup?

Using MakeMKV will be a single step to making a .mkv file from a disc. Like the process you linked to, you'll end up with an exact copy of the movie in a .mkv container.