We won’t spoil the surprise, what happened after 6 hours can be seen in the short video attached below
Namely, it is a reaction of phosphoric acid which happens when Coca-Cola is mixed with milk.
When the molecules of phosphoric acid come in contact with milk, they get more density leaving the remaining liquid lighter; therefore, in accordance with the laws of physics, the liquid rises to the top of the bottle.
Basically at the end of the process, the solid material at the bottom of the plastic bottle is something like sour milk or milk with more acidic soda (sodium hydroxide).
The pH value of Coca-Cola, depending on the country where it is produced, ranges from 2.5 – 4.5 and the value of milk is 6.7.