Yes onions can really absorb bacteria that is around it.
But if you mean the myth about the onion absorbing the bacteria around you and preventing you from getting sick then that is just a myth.
But if bacteria is present then the onion can absorb that bacteria and then when you eat the onion it could make you sick.
But you would usually notice that the onion has gone bad and not want to eat it.
Chopping onions in advanced can be done as long as you store the chopped onions properly.
If you need to chop onions in advance make sure you put the chopped onions in an airtight container or an airtight bag and then place them in the refrigerator.
The onions that are cut in advanced should keep in the refrigerator for up to 5 days before they go bad.
It's best to just wait and cut the onions the same day you're gonna use them though for the freshest onions.
But if you need to cut onions in advanced you can do so.
Onions do not go poisonous once they are cut and stored.
You can store the onions in the refrigerator or freezer as long as you keep the cut up onions in a freezer bag or in an airtight container.