For graphics, the problem to be solved is that the N64 compiled code is expecting that if it puts value X at memory address Y it will draw a particular pixel in a particular way.

Emulators solve this problem by having a virtual CPU execute the game code (kinda difficult), and then emulator code reads the virtual memory space the game code is interacting with (easy), interprets those values (stupid crazy hard), and replicates the graphical effects using custom code/modern graphics API (kinda difficult).

This program is decompiling the N64 code (easy), searches for known function calls that interact with the N64 GPU (easy), swaps them with known valid modern graphics API calls (easy), then compiles for local machine (easy). Knowing what function signatures to look for and what to replace them with in the general case is basically downright impossible, but because a lot of N64 games used common code, if you go through the laborious process for one game, you get a bunch extra for free or way less effort.

As one of my favorite engineering phrases goes: the devil is in the details

Whenever I replay OOT I never have a problem with Navi. She rarely hard interrupts, usually just a short tone and flashing C button that goes away after a few seconds. The voice lines only trigger if you press the button to call her, in most cases the hints she gives are genuinely helpful, and stays out of your way for the vast majority of the game.

Fi from skyward sword though… Far worse because she does interrupt gameplay, often repeats what the last dialogue box just fucking told you, and takes several dialogue boxes to tell you what Navi would have taken one to do. I’m glad they significantly overhauled her interactions in the HD release but I’m still going to be hesitant to play that game again