if i play fizz to my base and as part of the ability play the spell bushwhack from my trash. Does the effect of Bushwhack work retroactively reading my fizz or does it only effect units after its played?

To determine how Bushwhack interacts with Fizz, Trickster, it is important to note that the effect of Bushwhack is not retroactive; it applies to the game state at the moment the spell resolves.

Because the spell enters the chain as a new item and resolves after Fizz has already entered the battlefield, Fizz is fully present on the board when Bushwhack's effect executes. Any friendly unit on the board at that time (including Fizz) will not be affected by Bushwhack. Step-by-Step Sequence

Play Fizz: You play Fizz from your hand. He enters the chain as a pending item.
Fizz Finalizes: Fizz enters the battlefield (your base).
WYPM Trigger: Fizz's "When you play me" (WYPM) ability triggers and is placed on the chain.
Targeting/Choice: You choose Bushwhack from your trash to play.
Chain Resolution: The chain resolves (LIFO - Last-In, First-Out). Fizz's WYPM ability resolves, and you play Bushwhack from your trash (paying its Power cost).
Bushwhack Finalizes: The spell enters the chain as its own, new item.
Bushwhack Resolves: When the chain reaches Bushwhack, the spell executes:
    Effect: "Friendly units enter ready this turn."
    Any units you already control on the board are not affected by this effect. As they do not 'enter' the board.
    Gold Gear: You then play the Gold gear token exhausted.

Why it does not work

The instructions on Bushwhack do not "look back" at when Fizz was played; they simply allow friendly units played that turn and after Bushwhack resolved, to enter in a ready state.

This interaction is consistent with how triggered abilities and spell resolutions function in Riftbound. As established in the Fizz, Trickster FAQ, playing a spell through Fizz's ability makes that spell a new, separate item on the chain, allowing it to interact with the board state created by Fizz’s entry.