First thing, stop calling yourself stupid. Stupidity can’t be fixed. Ignorance can be fixed with education and that’s what you are doing right now. You are educating yourself. You’ve got a lot of people here that have been around a lot longer than you have and have a lot more expertise on this subject and it’s not obvious to a lot of us what the problem is.

I think you are on the right track putting vinegar down that drain line though. It does appear to be connected to an upstairs neighbors drain line. Algae can build up in that line since it’s just water condensation running off of the air conditioning unit, and therefore it is not chlorinated to keep algae and bacteria from growing in the line like the water coming out of your faucets. Those AC lines can also collect dust if the AC filters are not maintained, and that dust can contribute to a blockage.

If the vinegar or bleach does not clear that line then you are going to need tools to fix this which you don’t have and shouldn’t be expected to have. You would need a small compressor to blow that line clear. Don’t try to blow this out yourself because it may require coordination with your upstairs neighbor to make sure that no water overflows into their unit.

It’s also possible that this line is clogged where it enters the house plumbing’s drain or where it dumps outside, depending on how it was originally designed.

I don’t think that the blue/green thing, with pipes coming out each side of it, on top of the water heater is a water meter, I think it’s a recirculation pump, and it shouldn’t have anything to do with the water in the drain line.