If you are a supergiant like Citadel you can simply acquire positions which are so large they are the equivalent of someone's portfolio (or put another way, Citadel sees a huge block of equities packaged up, looking to move and bids on it).

It would be like if you needed to acquire 100M shares of Apple -- usually a broker will fill that large of an order from several sources OR if some fund has the full liquidity, it can be filled from that single position (and thus acquiring the fund's portfolio position). Now repeat this for their ENTIRE portfolio.