Sifo Dyas Explained
This is one issue that has actually been explained in Star Wars canon. In a book or comic, they explained that Sifo Dyas was strong in foresight in the Force and sensed the battle that was to come with the Separatists. He became obsessed with it and went out on his own to Kamino and ordered the creation of the Grand Army of the Republic.
Eventually, Darth Sidious and Count Dooku heard of his plan and had Dyas killed and took over the project themselves. Unfortunately for someone who watches the movies only, they have no knowledge of this and this issue is one loose thread that remains.
Sifo Dyas is mentioned a couple of times in Attack of the Clones but there is no answer given for why he ordered the creation of the Clone Army. A viewer who watches just the movies could think he was coerced, forced and then murdered, or was acting under his own volition. There is no explanation whatsoever, and it is one loose end that doesn’t get resolved in any future Star Wars episodes.
This could be resolved with just a simple line of text in The Rise of Skywalker. Maybe there is a scene where Rey is reading the Jedi texts aloud and that part is mentioned. It could be very brief and would be an easy way to tie up one of the biggest loose ends in Star Wars.