Almost all of the "custom" chips on these arcade systems through the 90s were not fully custom, they were gate arrays, sort of a fixed form of FPGA. You'd sele

Almost all of the "custom" chips on these arcade systems through the 90s were not fully custom, they were gate arrays, sort of a fixed form of FPGA. You'd select from a few templates depending on how big a die you needed, or any distribution of special features like sram blocks, etc. And on this die would just be a giant grid or sea of gates, not wired together besides power and clocking. Then you would use the vendors tools to take your logic (in schematic format or very early HDL like ABEL) and they would generate a routing solution using 2 metal layers of interconnects to wire all the gates together. Then you could order a few tens of thousands of chips, and the fab would put your 2 metal layers on top of the silicon and package it. The cost was honestly not bad at all. NEC's prices ran about $50k of tooling (not including any software or design tool rental) which is peanuts. The per unit cost was higher than fully custom silicon though. Companies offering gate arrays were NEC, Toshiba, Ricoh, Fujitsu, Sharp, and so on. By the early 2000s this started to wane and NEC started pushing more incentives to keep the business going but now they much less common. There were services that would take a working FPGA design and handle the conversion for you. Gate arrays were a stepping stone in the journey of integration and consolidation. If you want to read more, look up a NEC Semiconductor Solutions Databook from the 90s.