Can you react to playing a spell like Cull the Weak, and does playing cards in general create a chain?

Ruling: Yes, playing a spell like Cull the Weak creates a chain that you can cast a reaction to. All types of cards, triggered abilities, and activated abilities create chains.

Nuances:

• Permanents (gears and units) resolve immediately with 0 reactions upon being finalized onto the chain - opponents cannot react before the permanent itself resolves
• "When I'm played" abilities of permanents can be reacted to, but not the permanent itself
• ADD abilities (tapping/recycling runes, legend abilities, seals, etc.) also create chains but cannot be reacted to