Question about Nintendo Switch - Grey

A normal power bank works.
With 5V/2.1A, the battery level of the console in the handheld can be maintained at its level. However, it should be noted that the specified 2.1A output can be supplied at 100% charge level of the powerbank. When the powerbank battery level drops (on mine approx. from 80 - 70%; with Zelda), the power of the powerbank is no longer sufficient to charge the Switch battery during use.

However, this should not be a problem, as the runtime is still extended by quite a bit. In addition, the battery can be charged when the console is in standby (albeit rather slowly).
As long as you don't play for several hours at a time, a normal 5V/2A powerbank will do the job just fine.

To whoever (for whatever reason) calls themselves Digitec: USB-c can go up to 15V with the voltage, depending on the device. Therefore, a cheap cable that doesn't manage this properly can destroy 5V devices. Razer has announced a 15V powerbank (for their new notebooks with USB-c charging port), otherwise I don't know anything about powerbanks with > 5V.

As far as I know, powerbanks (if USB connection) always have 5v. It depends on the power (watts) - and since the voltage may not be changed with USB (always 5v), the amperage (A) is decisive. In my opinion, powerbanks usually don't offer much more than 2A per output - at least I don't know of any that deliver much more current.
The storage capacity, in turn (given in mAh or Ah), says something about how much current is contained in the bank. The Switch has a battery with just over 4000mAh. This means that if the powerbank has 10,000mAh, in theory this is a little more than twice as much as the Switch battery. In reality, however, the transfer process from powerbank to device is lossy. If you also play games while charging, the charging time is extremely prolonged on the one hand, and the powerbank also runs out more quickly on the other.
When making a purchase decision, I would look for a powerbank that has an output of >2A. Also note that you need a cable with a USB C connection for the Switch.

A car charger for the Switch only achieves 1.6A at 5V. All a bit confusing