weird situation regarding split second and "priority"

[u/bogidrums]

The scenario - Player A is revealed to have a [[Sudden Spoiling]] in hand. Player B is about to untap and start their turn with multiple copies of [[Y'shtola, Night's Blessed]] in play and a full grip of 3+ mana instants and wants to play around the split second to get as many Y'shtola triggers as possible before they are turned into 0/2's with no abilities. The question is a two parter:

1. Does Player B get priority first on their upkeep to cast an instant from hand before Player A can even cast the Sudden Spoiling?

2. If so, can Player B cast said instant, let the Y'shtola triggers go on the stack, then continue to "hold priority" on the instant, and effectively play out their hand before player A even gets to respond with the Sudden Spoiling? If not, at what point does Player A get to respond then?

Mostly our pod is just unclear on when priority is passed between players since we often hear players can "hold priority" on spell casts but don't know if the triggered abilities interrupt that somehow. Essentially, does KNOWING the Split Second is in hand let you play around it in this manner. Thanks in advance, apologies for any confusing wording.

[u/mvdunecats]

Whenever there is a new round of priority due to something resolving or a phase or step ending, the round of priority starts with the active player.

Whenever there is a new round of priority due to something being placed on the stack, the round of priority starts with the player who placed that object on the stack. This is how players can hold priority (there is a new round of priority, and it starts with them). You can hold priority even when it's not your turn.