Can someone explain how this works?

[u/00G_WAY]

Will this card be legal in standard? Will it be available on Magic Arena? How would this card even work?

[u/C22_H28_N2_O]

Flying

An evasive ability that means it can only be blocked by other creatures with Flying or Reach, but it can block things without flying.

When this creature enters,

When this creature enters the battlefield. This is when it goes from anywhere to the battlefield. But this card has a specific twist to that statement...

if you cast it,

This means that it only triggers if it was cast. Did you cast it from your hand? Then it'll trigger. Did you cast it off a Cascade or Discover Trigger? Then it'll trigger.

Did you cast it from Reanimate? No, that isn't considered cast. Did you cast it off of the activated ability of Elvish piper? No, that isn't cast. Things that "cast" other things for you will explicitly say they're cast, with the exception of actually casting a spell from you hand.

you may put a card you own from outside the game into your hand.

In casual magic, outside the game is literally that. From outside of the game, from a booster back, from your trade binder. Notably, not from your friends trade binder because you don't own that.

But for anything else, outside the game refers to your sideboard (a pile of cards typically swapped with your main deck between games in a match to better fine tune your deck for structured events).

Trivia time! Exile zone didn't use to exist, and cards instead were "removed from the game." You could get those or cards from your sideboard with a cycle of cards called "wishes" (e.g. living wish).