As written in multiple forums, IoTA tips are chosen using the random walk method starting at the milestone transaction.
The milestone transaction will have the reference to the old transactions. Tip transactions are added after the milestone transaction and there is no link or reference from milestone to tip transaction but tip to milestone transaction is there directly or indirectly.
How can the random walk method start from the milestone transaction and move to the tip transaction when there is not reference.
In simple way,
a<-b<-c ; in this a is milestone and c is tip
How can we move from a to c, when there is no reference? or Any other method is followed to choose the tip.