Clue is an artifact type. Even though it appears on some cards with other permanent types, it's never a creature type, a land type, or anything but an artifact type.

The extort ability doesn't target any player.

Some spells and abilities that investigate may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. You won't create any Clue tokens.

Some abilities trigger "whenever you sacrifice a Clue". Those abilities trigger whenever you sacrifice a Clue for any reason, not just to activate a Clue's activated ability.

If you haven't gained 4 or more life by the time an end step begins, Syndicate Heavy's last ability won't trigger at all.

You investigate just once, no matter how much life you've gained past 4.

Syndicate Heavy's last ability looks at how much life you've gained in the turn, even if it wasn't on the battlefield when you gained life. It doesn't care if you also lost life, even if you lost more life than you gained.

You can't sacrifice a Clue to pay multiple costs. For example, you can't sacrifice a Clue token to activate its own ability and also to activate Alquist Proft, Master Sleuth's ability.

The amount of life you gain from extort is based on the total amount of life lost, not necessarily the number of opponents you have. For example, if your opponent's life total can't change (perhaps because that player controls Platinum Emperion), you won't gain any life.

You may pay a maximum of one time for each extort triggered ability. You decide whether to pay when the ability resolves.

If an effect refers to a Clue, it means any Clue artifact, not just a Clue artifact token. For example, you can sacrifice Wrench to pay for Alquist Proft, Master Sleuth's activated ability.