Yes, UHS-I SD cards usually work in UHS-II memory card slot in camera. Also UHS-II SD cards usually work in camera UHS-I memory card slot.
UHS-I and UHS-II memory cards usually have the same dimensions, the difference is in the number of pins. While UHS-I card has 1 row of pins, UHS-II card has 2 rows of pins.
If you use UHS-II SD card in UHS-I slot, it will not work at its maximum speed.
Card readers work in the similar way. Most of SD card readers support UHS-I. If you put UHS-II card to UHS-I card reader, it will probably work, but it will be limited to UHS-I speeds.