You are not stupid, just you do not have the experience or the knowledge to repair an amplifier.
IN such a condition, you have no chance to repair this amplifier without any technical information you need to understand how it works.
So, the minimum necessary is the schematic diagram of this amplifier.
Your video is totally ridiculous and I recommand you to delete it from youtube.
About safety, what is important are safety glasses to protect your eyes if something explode.
You do not provide us with any usefull information as the type of the audio module (STK ?), nor information on a possible relay, nor inputs and outputs, ....
Based on photos found on the internet,
- there is no loudspeaker protection relay
- There is no output preamp / power amp input .... but, there are record outputs which could be use to check if input selector and preamp are working or not.
To troubleshoot low frequency amplifiers, you need very few instruments: a multimeter and a signal tracer / injector signal (like this one
https://www.velleman.eu/downloads/0/illustrated/illustrated_assembly_manual_k7000.pdf ). No need for power supply, oscilloscope, or function generator.
A variac is very useful, but a current limiter with 100W incandescent lamp in series can suffice.
With regard to the failure, it is either a power supply failure or a fault in the input selection circuits or preamplifiers.
The power amps look ok.