I have Detonate in my hand. My friend has Darksteel Plate out (and no other artifacts). They are at 3 life so if I can target it and get the damage I literally WIN THE GAME! Can I and Do I?
Yes, Detonate can target any "artifact with converted mana cost X", even if it has indestructible (C.R. 601.2c, 108.1). If the target is still legal, Detonate will still deal the damage when it resolves, even if it doesn't destroy that artifact (Detonate doesn't say, for example, "If that artifact was destroyed this way, Detonate deals...".) (C.R. 608.2b-c, 101.3; see also C.R. 609.3). If your opponent is brought to 0 life this way, they will lose the game and you will win (assuming it's a two-player game) (C.R. 117.3b, 117.5, 704.5a, 104.5, 104.2a). See also this thread.
Can I target an Indestructible artifact with Detonate?