In a saner world, it would be implicit in the USB-C. As it is, you know that it supports 5V.
It might also support higher voltages (up to 20V), though if it doesn't specify I wouldn't assume. If you need higher voltage, you'll have to look for a battery that specifies it. (And hope that the package isn't lying.)
Every powerbank vendor I've seen gives the battery rating at 3.6/3.7V, so USB implying 5V gives us misleading values. So a 20,000mAh powerbank is 20*3.6V=72Wh, not 20*5=100Wh.
I've never seen a vendor that specifies 3.6V, the best you can hope for is that they give mAh and Wh so you can derive it yourself. If we need the Wh rating to keep the mAh rating honest, the mAh becomes redundant before the Wh does.
It might also support higher voltages (up to 20V), though if it doesn't specify I wouldn't assume. If you need higher voltage, you'll have to look for a battery that specifies it. (And hope that the package isn't lying.)