It’s a combination of IP and deep institutional expertise. The 5G standards plus other important protocol documentation, for example, are on the order of fifty thousand pages, built on decades of experience with edge/2g, 3g, and LTE. That’s just the documentation on the protocol, the real secret is in the mixed signal ICs that require custom cell libraries which Qualcomm/Broadcom work with fabs to develop for their own use. The only other company of note in this field is Apple which bought the initial IP from Intel (which bought it from Infineon, another IC manufacturer), so we’re talking about something so technically complex that only the deepest pockets and expertise can make any headway. When Apple bought Intel’s modem IP, over two thousand employees transferred with the deal, to give you an idea of the scale.

That’s just the radios, which is their bread and butter. A lot of their other products have similar barriers to entry.

As the sibling comment noted, FPGAs aren’t even in the running. Ignoring their power consumption, the biggest FPGAs only have a hundred thousand or so logic elements. While its not easy to map that to number of transistors per se, even a legacy nodes are capable of much more complex designs than you can fit on an cutting edge FPGA. This really makes a difference even at the lower end because you have to get the timing right between all the different parts of your logic, and making everything smaller gives a lot more room for error (its a lot easier to put delay lines than to reconfigure a section of your design to fit closer to another section).