Ok i see you are a bit confused, let me try to explain

Lets call your units MAIN unit and FOODER, you use X rarity fairies when you give FOODER to you X rarity MAIN unit

So lets say i have a silver archer i want to level up (Soma) so give her 1 silver fairy, that rises the gained XP by 2x, another silver fairy will be 4x (2x for each fairy 2x2 = 4) and 3 silver fairies will be 8x ( 2x2x2 = 8)

IF you want to give her (Soma) another rarity of fairy (bronze, gold, platinum, black) they will only rise the XP by 1.5x each, instead of 2x if it was a silver one so 1 fairy is 1.5x, 2 fairies will be 2.25x (1.5 x 1.5 = 2.25) 3 fairies will be 3.375 (1.5 x 1.5 x 1.5 = 3.375)

Regarding the platinum armor, no, it doesn't matter to whom you feed it, it will always yield 1000 XP base, the bes use for those armors is using it with 3 corresponding fairies so you get 8x 1000 = 8000 XP on that unit

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I hope i made it clear... to some extent