Originally Posted by
aAlouda
Using mana from the enviroment for magecraft is something most proper mages do, Shirou cant do it because he sucks, but it's the common way to use magecraft, even non-gifted mages like Kiritsugu and Aoko can do it. We also see Rin explicitly do it when summoning Archer and more notably at at great potency when Fighting Sakura, where repeatedly absorbed the mana in the cave before unleashing it through the gem sword to destroy the familiar, then opening a path to a paralel world and using the mana from the cave in it too, doing it for every one of Sakura's familiars.
Yes? That just means their specialty is storing magical energy in other objects. Rin even specifies that she isn't limited to using her own magical energy, and with some difficulty can even store things that aren't magical energy in her jewels.