Both. And there's quite frequently a team of hundreds involved; the latest Intel flagship processor will probably have had over a thousand people involved in design decisions somewhere (especially if you count technical input from the foundry, which is vital if you're using a new manufacturing process).
Generally the process involves:
- high level architecture design, by humans
- implementing the design in Verilog and/or VHDL, by humans
- translating that into logic gates, by software
- placing the gates and routing wires between them (software with constraints and guidance by humans)
- analyse the result (software) and review key areas for improvement (by humans)
- translate gates to images for photomasks (represented in the "GDS II" file format)
- generate test data (software-assisted humans)
- manufacturing and test process (mostly automated)
Edit: there are also lots of things which up the transistor count without having to design them all individually; SRAM, for example, is a large grid of repeating elements. The A7 will have a substantial fraction, maybe a majority, of its transistors devoted to L1 and L2 cache.