Usually generated by a dedicated PMIC (often a TPS-style controller) to power the EC (Embedded Controller) and start the boot sequence. Secondary/Run Rails: +1.2V (DDR4): Power for memory modules. +1.0V / +1.8V: Core logic and PCH standby voltages.
Let’s put theory into practice. Assume a dead LA-H103P board. la-h103p schematic