[Question 1.23] How do the terrain sprites work?

In 1.15 all you had to do to change the sprites and get HD textures for your machinery was to swap out item_0.png and block_0.png with versions with better textures in HD.

Now, that no longer works. The machines look the same even when I overwrite the files in the jar in the mods folder. Furthermore, there are additional files with repeating textures in them in the mods folder. What is going on here?