Premium Quests are unlocked as one-off licences. A licence opens a Premium Quest for exactly one of your soul matches, permanently. It is a per-pair unlock, not a personal library; sharing the same Quest with a different match means a separate copy for that pair.
Unlocking one with a match works like a gift: you assign the copy to them and they accept it. A gift that has not been accepted does not unlock anything yet, and you can withdraw it before it is accepted.
Premium Quests are not part of any subscription plan. Subscriptions buy talk time; Premium Quests are their own thing, paired to one conversation that matters.