The triggered ability of The Myriad Pools will resolve before the spell that caused it to trigger.

The target permanent copies what is printed on the original card, plus any choices that were made when casting the spell.

If the copied spell has {X} in its mana cost, X is still 0 for the purposes of determining the mana value of the permanent on the battlefield.

Non-copy effects that have already applied to the target permanent will continue to apply to it. For example, if it's a creature, +1/+1 counters on it will still affect it.

Because the target permanent isn't entering the battlefield when it becomes a copy of a permanent spell, any "When [this permanent] enters the battlefield" or "[This permanent] enters the battlefield with" abilities of the copied spell won't apply for the target permanent.

Cards with the ability word "descend N" have abilities that care if you have at least N permanent cards in your graveyard.

Some descend triggered abilities include intervening "if" clauses (i.e. "if you have [four or eight] permanent cards in your graveyard" in the middle of the ability). Each of these abilities checks your graveyard at the moment it would trigger to see if it does. If you don't have the required number of permanent cards in your graveyard at that time, the ability doesn't trigger at all. If it does trigger, it will check again as it tries to resolve. If you don't have the required number of permanent cards in your graveyard at that time, the ability won't resolve and none of its effects will happen.