To repair something, you must know how it works. If schematics or service manuals are not available, you are left to guessing.
To guess, Check power supplies, 3.3V, 5V, 12V... whatever is present in the power pins of many ICs. Next, see clocks, data lines activity in memory chips, microcontrollers.
Next follow the communication lines to the external world, there should be chips handling the OBD protocols; identify traces to such communication IC; check if there is data out/in at its corresponding pins. Hard to probe with boards coated.
If ICs numbering can be followed to their data sheets, it is of great help.
I suppose the manufacturers of such ECUs do not want tampering with their units and that is why are secretive in their design. It is a safety issue and lawyers are always ready to sue and profit from unauthorized servicing in the age of dispose, buy new.