How did you - on that 2k+ developers project - deal with the fact that sometimes someone is working on a change in a feature branch (a) that the an other develo

How did you - on that 2k+ developers project - deal with the fact that sometimes someone is working on a change in a feature branch (a) that the an other developer also needs for her feature branch (b)? Would you move the commits that need to be shared away from the features onto a seperate branch (c) and then .. ? I am really curious how you handled these sort of situations at that scale!