What is this extra line coming in connecting to the drain under my bathroom sink?

Dishwasher drain line?

i say yep

Maybe an air conditioning drain line, some cities you can’t drain to the outside.

What other things do you have around?

Pretty sure this.

Second floor bathroom sink. Definitely not dishwasher. AC line is possible; I only see one drain line outside for two units, and proximity is reasonable. Not sure why you’d drain one outside and one inside though.

Condensate drain line. Replace those supply lines with new ones when you can. They will break on you.

Yup, ac condensate drain

Primary drain for your AC.

AC drain

Likely not two different lines for two different units.

Both drain to the sink. If the sink line clogs or if the line from either unit to the sink line clogs before the junction, then it drains to the backup line which sends the condensate outside. If you have anything coming out of that outside line, there's a clog somewhere.

Ac drain

Who installed the plastic shut offs and supply lines. Accident waiting to happen and lots of water.

Yep