Strata four storie apartment block, a main floor toilet bubbles when a forth floor directly above, shower or washing machine empties... all plumbing goes to the underground parking

Bubbles indicate a partial blockage downstream from the main floor toilet. A "bubble" indicates a clogged line while a "gurgle" indicates a blocked vent. Let me explain. When you flush the discharge drains down the pipe until it hits a partial clog. Then it rebounds back sending a "bubble" of air ahead of it. This is the "bubble" you see. The solution is to have the drain bline snaked and cleared. Good luck. Tom