After rewatching the scene where all three of them were seated at the table, it was not revealed how the survivor drugged both Joon-woo and Yu Bin, but there were camera shots that could suggest how the drugs were administered. Sorted chronologically, here are the shots.
Camera shot #1
Without the help of a tray, the survivor carries three empty cups by having his fingers in the insides of the cups, and places them on the table.
Camera shot #2
The survivor pours water into a cup and hands Yu Bin the cup. Instead of three cups, only two cups can be seen.
Camera shot #3
Joon-woo opens a sealed can of SPAM and Yu Bin opens a sealed can of corn. Both are later seen sharing the food but not with the survivor.
Possible methods of drugging
- From #3, the drugs could not have been administered via the cans of food since both of them were sealed. However, both Joon-woo and Yu Bin were not seen how or where they had gotten the utensils from. The utensils could possibly be tainted beforehand.
- From #1, because of the fingers, the cups were possibly tainted from the moment the survivor brought them to the table. From #2, the cup handed to Yu Bin remained in frame for nearly 4 seconds, which could imply that the cup was indeed 'special'.
In my opinion, the second method is highly probable and there are some clues to support this. In Korean dining culture, it is often impolite to dip your fingers into a cup when serving to house guests. Typically, there should be a serving tray to bring cups and serve, especially in a young Korean family. The survivor choosing not to use it is a clue.
The survivor also confessed that he had to hide from his wife/family to smoke. This can also explain that he knows a thing or two about deception to hide his motive from people.
And I suppose the drug used is related to sleeping pills, quite common in South Korea where insomnia and other sleep disorders are prevalent. The survivor was seen standing at an area where baby milk bottles were prepared to feed an infant. This can explain the need for sleeping pills where the survivor wanted to catch some sleep at specific hours of the day, after taking care of an infant.