Question about Nintendo Game & Watch: Super Mario Bros.

> can i charge the device with a USB-C out USB-C in cable?

Yes

> Can I use the iphone 15 cable?

I have Android but I would say yes

> Can I use a 20W USB-C adapter?

Yes, I have used any source, from computers, to power banks to 165W USB-C adapter chargers and they all work

Yes, shouldn't be a problem.