FPGA Element Selection
I would like to know how FPGA manufacturers decided what elements to include? I.e., how is the internal design of the FPGA determined?


A major part of this is telemetry. Xilinx collects data from their users through a feature called WebTalk, for instance -- this data includes information on the utilization of fabric and routing resources, and is likely used as an indicator of what blend of resources should be included in future parts. Intel (previously Altera) has a similar feature under the name TalkBack, and other manufacturers probably have similar functionality as well.

Beyond that, FPGA makers will often work closely with large customers in developing designs, and will take feedback from those customers about unexpected limitations of their parts.