Construction Arsonist 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

When Construction Arsonist enters, you may choose an instant or sorcery card in your hand. If you do, that card perpetually incorporates and gains "When you cast this spell, it deals 2 damage to each opponent." Whenever you cast a multicolored spell, Construction Arsonist gets +1/+1 until end of turn for each of that spell's colors.