If an ability instructs a creature to explore, its controller reveals the top card of their library. If it's a land card, they'll put it into their hand. Otherwise, they'll put a +1/+1 counter on that creature, then choose to either leave that card on top of their library or put it into their graveyard.
Nicanzil's first ability won't resolve until after you're done exploring. You may put any land card from your hand onto the battlefield, including the land card you explored (if it's still in your hand) or one that was already in your hand.
In some unusual cases, noncreature permanents may explore. For example, if the creature card returned by Defossilize is somehow not a creature once it's on the battlefield, it can still explore. You'll take all the same actions, and you may end up putting a +1/+1 counter on the permanent. (Note that some effects target a creature, and those effects would still require a legal target to have it explore.)
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If a resolving spell or ability instructs a specific creature to explore but that creature has left the battlefield, the creature still explores. If you reveal a nonland card this way, you won't put a +1/+1 counter on anything, but you may put the revealed card into your graveyard. Effects that trigger "whenever a creature explores" trigger as appropriate.
Nicanzil, Current Conductor's triggered abilities care about the characteristics of the card as it was revealed while a creature explored. It doesn't matter what happened to the card after that.
Some spells or abilities might cause a creature to explore multiple times in a row. If you reveal a nonland card when a creature explores and leave it on top of your library, then the creature explores again immediately afterwards, you'll reveal the same card again.
Once an ability that causes a creature to explore begins to resolve, no player may take any other actions until it's done. Notably, opponents can't try to remove the exploring creature after you reveal a nonland card but before it receives a counter.
If no card is revealed, most likely because that player's library is empty, the exploring creature receives a +1/+1 counter.
If a creature leaves the battlefield before an effect instructs it to explore, it still explores. Abilities like Nicanzil's that trigger whenever a creature you control explores will still trigger.
