Does Teemo Strategist on defend effect happen during finalisation or resolution? I want to know whether I can react to Teemo's on defend effect after targets have been chosen.

Yes, you can react to Teemo, Strategist's "When I defend" ability after its target has been chosen.

According to RiftJudge FAQ #7962:

• The triggered ability is added to the chain, and targets are chosen as the trigger is placed on the chain (during finalization).
• Once the ability is on the chain with its target locked, your opponent has the opportunity to play [REACTION] cards before you begin resolving the ability (revealing cards).

Summary of Timing:

1. Trigger: Teemo gains the "defender" designation (e.g., when an opponent moves a unit to his battlefield).
2. Finalization: The "When I defend" triggered ability is added to the chain. You choose the enemy unit target at this time.
3. Priority Window: Because the ability is now on the chain, the state is CLOSED, and players have an opportunity to play [REACTION] cards (e.g., to move a unit, buff a unit, or remove Teemo).
4. Resolution: Once all players pass, the ability resolves: you reveal the top 5 cards and deal damage to the chosen target.

This is consistent with the standard FEPR process (Finalize, Execute, Pass, Resolve), where choices like targeting occur during finalization, and reactions can be played after the item is finalized but before it resolves.