Nintendo switch load with Power Bank?

I spy on a nintendo switch and have heard that you can't take any power bank to load them therefore my question is if it is enough if my power bank has an output with 5v and 2.1A

Yes, it should fit

But does the battery charge as well or does it only make sure that it does not sink?

Your powerbank will deliver just as much as the power supply, so if the power adapter loads the switch, the powerbank will do the same.

5 volts is normal. 2.1 amps is very much.