I don't fully understand the use of amiibo in a run. To my understanding you want to remove the rng from getting a banana in Hyrule castle. I just don't understand if you have to use a certain amiibo or if there is still some rng and you just use a random amiibo. Thanks in advance.

Each "The Legend of Zelda" amiibos have specific drop tables. So for Any% runners use either of the Toon Link or the Mipha amiibo as they are the ones dropping the fishes that you need.

Yes there is still rng but you have much greater odds