Usually the gully trap will be the first one that will leak below when it is not properly waterproofed..

Looks like your toilet is covered with plaster ceiling.

So your downstair neighbor got leaking from the floor, is obviously from your toilet.

From my experience, got a few potential point

1. Water floor trap side dun have good finishing, hence water go in seep inside

2. The WC dun have proper water proof at the bottom during installation

3. The floor dont have proper water proofing or floor cement crack due to floor expand from heat or minor shaking, so causing the water proof layer to tear off, hence got leaking.

Quick solution is using PU injection from neighbor toilet ceiling. They will drill  a few holes and inject the solution and hardened up.. so lesser mess to ur side..

If not u need to crack all tiles and redo .. bigger mess.. will cost u around 3k to do it

Thanks! In fact what is Gully Trap? And, which screw in cap means?

Update:
6 hours of Not Using Toilet, neighbor reports "Still Leaking/water dripping"!

What does it mean?

Another UPDATE:
Contractor just came to inspect this morning. And he reminded me about water bill, once check, Alamak... From 10 raised up to 28! Which means the pipe of the water source leakage!

Still, while turned of the valve of the bathroom, and without using water the whole unit, meter still running! So it's quite confirm of the Pipe inside the water leaking. Further more, the toilet is using Blue Pipe (they called it pipe hantu 🤣).

Btw, to ensure it's, they'll came tomorrow for "pressure test". Otherwise, the possibility might be bathtub drainage connectors leaking.

Update you guys then 😊