I've tested this as well by removing the power cables, including the 4-pin CPU connector and the 24-pin motherboard connector, and then nothing will turn on, but then I'll reconnect them and the fans will turn back on, meaning the cables are giving the motherboard power, but the monitor still...
I have just built my new and first ever PC using an ASRock B450M HDV R4.0 Motherboard, a Ryzen 3 2200G CPU, a EVGA GeForce GTX 1060 3GB SC Graphics Card, OLOy 16 GB (1 x 16 GB) DDR4-3200 MHz Memory, a Apevia AD 500 W ATX Power Supply, a Western Digital RE4 1 TB 3.5" 7200RPM Internal Hard Drive...