Season Pass is a pass you can purchase in game with GC that will grant you rewards as you progress through it. It starts upon purchase and there are different rewards for each level, which can vary from 30 to 50 rewards (currently).
Season Pass experience in Infestation: The New Z is not directly related to any other Experience stats in the game.
Though you gain the same amount of Season XP as you with Account XP, they do not necessarily follow each other, as Seasons come and go while Account XP is permanent.
You only start gaining Season XP after purchasing the Season Pass, and not before.
The amount of XP gained is also very different from the Character EXP, which is what you use to level up your character skill tree.
Season XP is also not affected by Double EXP boosts, from either account subscriptions or events.
With that in mind, here is some information on what gives you Season XP and how much.
Learning a Skin - 50 XP [ACC]
Purchasing GC - 1 XP per GC [ACC]
Victory - 210 XP [BR]
Mission Completion - 45 XP [ALL]
Player Kill - 28 XP [ALL]
Time Played - 12 XP per Minute [BR]
Zombie Kill - 4 XP [ALL]
Super Zombie Kill - 80 XP [OW]
Alien Zombie Kill - 110 XP [OW]
Toxic Zombie Kill - 100 XP [OW]
Slicer Zombie Kill - 125 XP [OW]
Conquer Clan Territory - 45 XP [OW]
Complete a Competitive Match - 125 XP [CP]
ALL = All Game Modes / ACC = Account-Wide actions / BR = Battle Royale only / OW = Survival and Arcade only / CP = Competitive