It's definitely not just a box with a keyboard and an emulator inside and it is specifically designed to be completely backwards compatible with all the old hardware. A fairly comprehensive list of specifications
here.
The following is a summary from people more knowledgeable than me:
This is NOT just a basic Z80 SBC Single Board Computer. It has many built in Audio interfaces, SD Card interfaces, i2c, SPI, i2s, Z80bus, RTC, Wi-Fi, RPi to do 'other' optional functions, More Advanced graphics modes and sprites, Multiple machine personalities, multiple cores, DMA controllers, a co processor similar to the Amiga, Banked memory, Multi format Joystick interfaces, A built in full keyboard, 2 monitor screen support, SID audio, MP3 and many other format audio playback, Upgradable FPGA based design.
It is an Official Sinclair Branded ZX Spectrum machine that has integrated hardware, Keyboard, expansion bus etc like a Real computer.
The Next is NOT like any other emulator or FPGA Synthesis. It incorporates the best of the older hardware and a lot of new Audio, Video and other functionality.