FWIW while I don't think it would actually happen, yes, Apple could "buy" Google. When companies merge the expectation is that the resulting combined entity will equal the two companies combined market caps.

Take theoretical company (Company A) worth $100MM and with a single shareholder (Shareholder A) who owns 100% of the 10MM shares of stock. They "buy" Company B which is also worth $100MM and has a single shareholder (Shareholder B) who owns 100% of Company B's 10MM shares. Company A executes this by issuing 10MM _new_ shares in Company A that will now be owned by Shareholder B. Company A now has 20MM shares and is worth $200MM because it has added Company B's assets, income, etc.

Of course in practice it frequently goes terribly wrong (ref: Microsoft & Nokia, Verizon & Yahoo, many others) and I don't think Google would actually have much incentive to go along with this. Just saying it COULD be done if both companies truly believed the outcome would be favorable.