Going to do ECU for work...
OP, ECU's are very complicated and car makers such as Tata go to Bosch, Continental, Westport etc. for ECU's. They just contract buy them.
You can't just pick it up and start to make one. It takes decades of experience and cross-discipline i.e. mix of EE, CE, MecE, ChemE to understand the engine thermodynamics, models and embedded systems design and programming, as well as knowing the many automotive standards and how to meet emissions standards as well.
Costs for a dyno, or chassis dyno and emissions monitoring equipment, calibration lab etc. are absolutely HUGE. Then you also have to purchase the standards SAE J 1949, ISO 26262 etc. and know them.
In reality it's a large team that would do ECU development.