The title is too simplified. The merger/acquisition will create a new entity in which Unity will own 55% of the total shares and 49% of voting shares. Unity CEO John Riccitiello will become CEO of the combined business, while AppLovin CEO Adam Foroughi will take the COO role.
This is basically a desperate play by AppLovin to prevent Unity from acquiring ironSource (its main competitor) and cutting it out of the mobile ads business.
According to the proposal the non-voting C-class shares are going to be given voting rights, so Unity stock holders will end up with the majority of voting rights too after that goes through.
In a lot of these acquisitions, it's not a becomes a subsidiary of b, it's often that all of the employees of A are transferred to B under something like a TUPE [0]. As part of these acquisitions, the leadership of the acquired team can be offered senior positions in the acquirer, e.g. in this case the CEO of AppLovin would become the COO of the new world.
Combine these two things and with a bit of luck and timing, AB corp now decided that A isn't in its best interests anymore and decides to sell that part of its business.
This is basically a desperate play by AppLovin to prevent Unity from acquiring ironSource (its main competitor) and cutting it out of the mobile ads business.