How does Fizz work and resolve? For example, if I want to target Frigid Touch in the trash, explain the chain.

To explain how Fizz, Trickster works when targeting a spell in your trash, we must look at the sequence of playing a unit and the subsequent resolution of its "When you play me" (WYPM) ability.

The Resolution Sequence

1. Play Fizz: You play Fizz from your hand. Fizz enters the chain as a pending item.
2. Fizz Enters Play: Fizz finalizes on the chain and enters the battlefield as a game object (Rule 356.2).
3. WYPM Trigger: Because Fizz is now in play, his "When you play me" ability triggers and is added to the chain as a pending item on top of any other items currently on the chain.
4. Targeting: When you put Fizz's ability on the chain, you target the spell in your trash that you intend to play (FAQ 6132).
5. Resolution: Once the chain resolves down to Fizz's ability, you execute the ability's instructions:
   • You play the targeted spell from your trash.
   • You must pay any required costs associated with playing the spell.
   • After the spell finishes resolving, it is recycled to the bottom of your deck (provided it was not countered).

Key Clarifications

• Targeting Timing: Contrary to the idea that targeting happens on resolution, targeting occurs when you put the ability on the chain (FAQ 6132). You must have a valid target (a spell in your trash) at that moment.
• Chain Usage: Units like Fizz enter the chain as part of the process of being played. Once they finalize, they leave the chain and become game objects on the board (Rule 356.2). The "When you play me" ability is a triggered ability that then uses the chain to resolve.

Note: I could not fully verify this ruling against the retrieved rules.