It's the wet bandits!

Say you were trying to drown the carpenter and flying ants.

Need to think about what you put down your drain. That is the reason your flooding.

Here’s a crazy idea: Grind up some laxatives and pour them down the drain. Then hold on for dear life.

Don't worry; the third one isn't you and nobody ever evicted someone for blocking each drain once.

Sounds like you have a lot of old pipes in your apt. time to talk to landlord

Sometimes if there’s a crack in the pipe tree roots can get in. It can be cleared (easily if your plumber has a machine they call a ‘snake’ here, that coils through the pipes chewing through any blockages), but if it is tree roots your landlord may want to think about getting it done annually to avoid the flooding. Less costly than replacing the pipes, which is the long term solution.

So - as an actual plumber/drain cleaner here, I can tell you something IS blocked. It could be the city sewer though which you cannot personally address, you have to call the city for. However, either way, the actual plumber who has been working on these issues should have been able to say WHY they are blocking up....and yes, these problems do happen all the time even if you put nothing wrong down there.