These are really cute and are as good as a starter or as a main. If you go for the main, I recommend that you have some sauce on the side (like pomarola), otherwise your guests might die of boredom.
Recipe left or right, what you need is cooled down risotto. It can be any type really, as long as it has no massive pieces of something in it (looking at you, broccoli). After you got your risotto sorted, you just add a cube of mozzarella in the middle and form a ball. Then you can let it rest for 30 min in the fridge or just immediately prep for frying.
It would work with rice as well. And yes, I have no clue how authentic these are. Probably not really, but they are really good!
Here you can find risottos on lindy.
Notes to the recipe - 50g per arancini - 160 degrees oil in a small saucepan works the best (I did batches of 4) - panko breadcrumbs make them even nicer