The abilities spliced onto the spell happen last, after all of that spell’s other effects. Notably, the Golem won’t be on the battlefield yet if the spell has an effect that affects all creatures, all creatures you control, or similar.

You reveal all cards you intend to splice at the same time. Each individual card can be spliced only once onto any one spell, although multiple cards with the same name may be spliced onto one spell.

If a spell is copied, choices made while casting it are copied, so the copy will have the same abilities spliced onto it as the original.

If the spell is countered, any cards you spliced onto it remain in your hand.

If all of the spell’s targets are illegal when the spell tries to resolve, it won’t resolve and none of its effects will happen, including those from cards spliced onto it.

The legality of a spell’s targets is checked only as that spell begins to resolve. If the spell this card is spliced onto causes its targets to become illegal while it’s resolving (for example, by removing them from the battlefield) you’ll still create a Golem.