Power On Voltages (Xbox One X)
If you have all the standby voltages, the next steps is to press the power button, which sends the Power Switch Enable pin low on the Southbridge (called the KIC, specifically the System Management Controller part of the chip called SMC), which in turn enables a few more regulators to power up the rest of the system, and send the white light on.
In short the Southbridge listens for the power enable signal going low from a button press, and enables many output regulators for the GFX, CPU Cores, Ethernet, HDD, ODD and more.
I will fill out more info for these stages when I get time but here is an overview.
U64 MP2949A PMIC (Memory)
U27 RT8071A VREG (For V_SOCPHY of Southbridge KIC)
U57 MP2853 PMIC (GFX/CPU Cores of U1 ANUBIS)
U68 AP2127KADJ VREG (For U1 rail V_FUSE of ANUBIS CPU)
U21 HDMI IC Enable
Through U34 op amp to enable Q3 to power M2 slot
U28 RTL8111HM Ethernet controller LED
U42 CAT6243DC VREG (to enable V_SB1P1 power rail of Southbridge KIC)
Turns on FETs U44 which turns on Q2 for ODD 12V
Enables U34 op amp to turn on FET U45 ODD 5V
Enables U34 op amp to turn on FET U56 HDD 5V
Enables U16 AP7365 VREG and FET U25 for V_3P3 for Southbridge KIC
Enables U51 MP8759 VREG for V_5P0 which goes to many, many places on the board