If the player controls an artifact land, they can’t choose to sacrifice that permanent for both the artifact and the land. If that’s the only artifact the player controls, they must sacrifice another land. If it’s the only land the player controls, they must sacrifice another artifact. If it’s the only artifact and the only land the player controls, they sacrifice just that permanent.

Structural Collapse targets only the player. The player can sacrifice an artifact or land with hexproof, for example.

The target player needn’t control an artifact or a land. Structural Collapse will still deal 2 damage to that player.