To comment a bit on the nice video;
1. CPU power cable, 4/8-pin 12V EPS is connected correctly.
2. "24 inch" - This wide cable isn't "24 inch", but instead it is 24-pin ATX. And it is used to power the MoBo and also supplementary power (up to 75W worth of) for PCI-E slots. Also connected correctly.
3. 8-pin PCI-E. Not connected, since obviously, there's no GPU in the system.
4. SATA power cable, that connects to the fan hub and then to the SSD. The bracket where the SSD sits in, is known as 3.5" HDD cage.
5. SATA data cable, connected correctly between SSD and MoBo.
6. USB 3.0 cable is also connected.
7. HD AUDIO is front panel 3.5mm mic/headphone jack, which is connected as well.
8. F_USB1 is internal USB 2.0 header. It provides 2x USB 2.0 ports on your case's front I/O.
9. Are the front panel connectors: power LED, power button (orange connector), HDD LED and reset switch. All are connected correctly.
10. CPU cooler. AIO pump is connected correctly to CPU_FAN header.
11. Where you ask, at 3:50, if you should have something there. No. That is PCI-E x1 slot and is reserved for PCI-E add-on cards, like sound card or additional USB/SATA/M.2 ports or wi-fi card.
12. While RAM retail package may not say specifically that RAM supports AMD CPUs, it could have the old label on the package. Official specs that i linked to you, do clearly state that RAM is compatible with AMD CPUs as well.
Actually, all RAM is compatible with any of the CPU. E.g DDR3 with Intel 4th gen or AMD FX-series; DDR4 with Intel 6th to 14th gen and AMD Ryzen 1000- to 5000-series; DDR5 with Intel 12th to 14th gen and AMD Ryzen 7000- to 9000-series.
13. Monitor is connected firmly to the MoBo's DisplayPort connector.
14. VDG connector is 3-pin 5V ARGB LED connector and connects either to the D_LED1 or D_LED2 header on MoBo (further reading from MoBo manual, page 20). When connected, fan/case RGB can be controlled via MoBo software.
Since you don't have much knowledge about PC components, i strongly suggest hauling it to PC repair shop for diagnostics. Since at this point, 2nd, compatible, known to work hardware is needed to find out which of the three is dead. Is it CPU, is it RAM or is it MoBo. These three are needed to see image on monitor. But since you do not have spare parts to swap out, there's nothing you can do further.
As of what went wrong during assembly, your guess is as good as any of us. Unless you were present when your friend assembled the PC and saw what he did.
Some possible faults:
* bent/broken CPU socket pin(s)
* ESD short to MoBo, which fried MoBo (happens when whoever assembles the PC, doesn't ground and keep themselves grounded when assembling PC).
* DOA (dead-on-arrival) components
Oh, when PC is powered on, DO NOT poke inside of it, like you did when checking if one cable connection was secure.