When you have sufficient funds, you can take over another megacorp in the city - whether they would want you to, or not. This action is available on the headquarters building or cybersite for each megacorp.
When you take over a megacorp, their reputation value for you goes immediately to 100 and all technologies they know are unlocked for you. This reputation may still change subsequently, depending on what actions you take, but it won't really matter because you own them.
The moment you take over a megacorp, all of their assets become yours. You gain all of their buildings, and with them all of their employees, whose wages you will begin paying immediately. Following a takeover you may wish to consider restructuring, and firing some of those newly obtained workers. Of course they will become homeless when you do, but that's just business!
When you manage to take over all other megacorps in the city, you will also own every building in the city and every inhabitant will work for you. There is no longer anyone else to advertise for, so the game ends - although you may continue to play on at this point if you like.