What Payment gateway does Fiverr/Upwork use?

On Fiverr, sellers' only withdrawal options are Paypal, Payoneer and Bank Options, However, how do these marketplaces integrate these payments method on their website to serve millions of sellers?

What I'm sure of

Integrate Paypay or Stripe(VCC) on a website to receive a direct payment from my clients.

What I'd love to know,

Since Fiverr pays their sellers through Paypal and Payoneer, is there a way to create similar website like fiverr, hold payments like Fiverr for few days until service is delivered, and then pay them via paypal or payoneer when funds get cleared?

It uses PayPal. You can pay via credit card too. Also I paid via Bitcoin also, but I don't know if this option is still available.

For integration and the service you need, I think there are few white label apps out there.

You should start from code canyon plugins, maybe you can find one for yourself for a freelancers site with payment holding features.

I think you have to contact payoneer and PayPal to get their api.