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External power for pc / notebook and DC to Dc converters need.

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Part one :

I have this Question ,

as we know , Most notebooks power adapters are 19.5V output.

While the CPU and GPU works on 12V and the SSD works on 5V . , so the motherboard should have two DC to DC power converters ,

one from 19.5V to 12V for CPU and GPU

and another From 19.5v to 5V for SSD nd harddrives.

mainly the notebook will never need 19.5 V .. so why dont they choose 12V Power supplies for notebooks in General and use 19.5 V power supplies ?

Why bother making two DC to DC inverters instead of just one ?

in case of 12V power supply , they already have 12V ready and just need a DC to DC converter to get the 5V for SSD.

keep in mind that internal desktop power supplies never outputs 19.5 V , it just outputs 12V and 5 V and 3.3 V ... now imagine this power supply external (lol) the place is not a big deal isnt it ?

so back to the main Q , Why all External power supplies used for PC outputs 19.5V and not 12V like in the internal ones ? it makes no sense.

Part Two :

Graphic Cards power ..

As we all know , most mid range to high end Graphics cards needs more power , and they come either with a 6 pins 12 v plug or 8pins 12V plugs for more power it needs (150 watts to 250 watts)

My question is : If I use an external power supply of 12v outputs and plugged it into the Graphic cards direclty , would it work ? or is it better to use the 19.5V notebook external power supply and add a DC to DC converter from 19.5 to 12V and then plug it into the card?

and in case the 12V works , does it need some kind of Board to make that 12V inline with the card voltage specs ? for more stabe 12V ? or is it Standardized and it will be within specs ?

some cards like the GTX 1070 needs 150 watts 12V input , and some like the 1080 , 180 Watts , and the 1080 Ti 250 watts.

I can find 150 watts external power with 12V output , but sadly anything more than this outputs 19.5 and needs DC to DC board ... example the DELL Notebooks power supply of 330 watts outputs 19.5 volts ...

Part Three :

some ATX power supplies come modular , and my question is : lets say I use just one 12V plug and ignore the other plugs , can I use the whole wattage from that single plug ? or is there a limitation of each plug watt?

They say it is single Rail (meaning watt is not divided between Vi) but I am concerned about the plug wattage and the wires if they can stand it or not.

Example : I need to power a graphics card of 300 watts .. and I use an ATX power supply with single rail of 350 watts ... the power supply has the motherboard connectors that I will not use , and has only one PCIe 6 pin plug for card .. but since I am not using it for mother board , can I use one port for the whole 300 watts or do I need to take 12 V from the Motherboard plug ?

I can use thicker wires for 300 watts , if the plug on the power supply can stand 300 watts ..

more over , on the PCB of the power supply itself , does it stand 300 watts since it is a single rail or must also be divided between plugs ?

Thanks for your replies .
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