@kostask5 Undervolting is safe, the worst that can happen it will be unstable or not boot. But
you aren't required to do it, it's a free extra - the low hanging fruit to get. Generally, changing settings in BIOS is preferred.
PBO is the extra boost on top of boost. If your CPU is not constrained by certain factors (power and temperature being one of them) then it will boost higher.
Undervolting can of course be applied to the GPU too - until you decide to overclock. Cooler and quieter, again better turbo possible.
You can undervolt regardless of CPU, you can only be limited by your BIOS not exposing this setting.
Again, going back to my i5 2400 with stock cooler: it would hit 3100MHz (base) and 70°C+ under heavy load and get really loud. I ran like this for years because I didn't know any better myself. Then I read about undervolting and experimented with it.
-180mV offset on CPU = constant +100MHz as turbo (running at 3200MHz all time now), lower temperatures and SILENT stock cooler. Not even under load was the fan spinning up like it did before and became indistinguishable from rest of the system (GPU remained the loudest component).