Most hardware is designed using a hardware description language (HDL) such as Verilog or VHDL. These languages allow some abstraction over the hardware that is produced so that it is more modular and easier to maintain. To translate these hardware descriptions to actual hardware, they can be passed through a synthesis tool, which generates a netlist referring to the specific hardware connections that will appear in the actual hardware.
Furthermore, high-level synthesis (HLS) is also becoming more popular, which allows for a much more behavioural description of the circuit using a standard programming language such as C or OpenCL. However, even designs written in these languages need to be translated to an HDL such as Verilog or VHDL and therefore also need to rely on a synthesis tool.