Question

what technique was used before he BIOS existed in early transistor and microprocessor based computers to boot or start them running code after power on?

Answer 1

In early computers, bootstrapping was not required. It started working once the power on, then start computing as similar as calculators
E.g., ENIAC
The earliest microcomputers and transistors had no bootstrapping when the CPU's power on. It would execute the code or instruction then the memory was cleared by resetting power. The boot process was revolutionized by the invention of the integrated circuit then the BIOS start function like the modern computers.