As Kaboom! resolves, if you have chosen multiple targets, those targets will be handled one at a time in turn order. Start with targets that are either the player whose turn it is (probably you) or permanents that player controls. You choose the relative order of targets within that group. For whichever target you select first, exile the top card of your library, then Soulfire Eruption deals damage to that target. Repeat this process for each subsequent target in the group. After that group is done, move on to targets that are either the next player in turn order or permanents they control. Again, you choose the relative order of targets in that group. Repeat the whole process for each other player.

If all the cards in your library are lands, this card deals zero damage.

Kaboom! : règles, fonctionnement et interactions (MTG)

Kaboom! is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Choose any number of target players or planeswalkers. For each of them, reveal cards from the top of your library until you reveal a nonland card, Kaboom! deals damage equal to that card's mana value to that player or planeswalker, then you put the revealed cards on the bottom of your library in any order.

How it works : Kaboom!

Original English text — Wizards of the Coast / Scryfall

#1September 9, 2022· Wizards of the Coast
As Kaboom! resolves, if you have chosen multiple targets, those targets will be handled one at a time in turn order. Start with targets that are either the player whose turn it is (probably you) or permanents that player controls. You choose the relative order of targets within that group. For whichever target you select first, exile the top card of your library, then Soulfire Eruption deals damage to that target. Repeat this process for each subsequent target in the group. After that group is done, move on to targets that are either the next player in turn order or permanents they control. Again, you choose the relative order of targets in that group. Repeat the whole process for each other player.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
If all the cards in your library are lands, this card deals zero damage.
No other card is named in this ruling.