A lot of the time people just plug in all their hardware without thinking twice. This works fine until you get to use custom hardware like riser cards because not all that sits fits.
The riser itself
The riser deliveres a maximum of 75W since that is also what a motherboard would supply. There are different variations of risers but the ones that are used for mining only extend the data lanes which is why you also need to power them.
Now, let´s get to the 3 most common connectors, 6Pin, Molex and Sata-power.
MOLEX-CONNECTOR
The Molex connector is usually seen on older hardware like hdd´s or to power fans.
POWER
The Molex connector has one 12V lane which kan deliver a maximum of 132W (12Vx11A=132W)
This makes it absolutly suitable for powering a riser and you should look out for risers with this connector. But try only using 2xMolex per cable to be on the safe side.
SATA-POWER-CONNECTOR
The Sata connector is usually found on newer Hardware (SSD´s ,etc)
POWER
The Sata connector has 3x12 Volt lanes which can deliver a maximum of 54W
(12Vx1.5A per line (3)= 54W)
This really makes it unsuitable for risers, because the GPU will try to use 75W from the riser. If you still have Sata powered risers, dont worry they have a lot of headroom (at least when using knowen brands like evga, bequiet!, corsair) which means it will work but you should only plug in 1 riser per cable(!) even if it is one of those 3x Sata power cables. If not your cables could melt or even catch fire.
6PIN(OLD)
The old 6 pin connector is now rarely found since it has been replaced with the 6+2 pin (below), but you will still find it with some psu`s so keep in mind.
POWER
The 6Pin uses the 12 Volt rail and can delivere a maximum of 75W
(12Vx2.083A per line (of which there are 3 of)= 75W
It exaclty deliveres what is needed to power a riser.
6+2 AND 8PIN
The 6+2 pin essentially is an 8 pin those 2 extra pins are just ground wires which help handleing the power. Very common on GPU´s nowadays.
POWER
The 8Pin too uses the 12 Volt rail and can delivere a maximum of 150W
(12Vx4.167A per line (of which there are 3)= 150W)
The risere most likely will have only a 6 pin connection, so what you are essentially plugging in is a 6Pin(which is fine) b u t those 6+2 cables are designed for higher power so there will be no problem.
Those are the 3 most common connectors, please keep in mind that even though it works to plug in 3x Molex or 3x Sata per cable it doesnt mean it is safe. Those stats above are the rated specs of these connectors. You most likley will never have to use an adapter most psu´s come with enough variation. If you have questions and/or like to add something please comment below thanks.