Unfortunately, for the moment, only the English manual is available: https://www.wortmanntelecom.de/images/wortmann-telecom/Dokumente-PDFs/Zertifikate/Xiaomi/Amazfit-Nexo/amazfit-nexo-user-manual-en.pdf

Yes :)

Specifications Case Diameter35.4 mmCase Thickness13.7 mm (bottom shell to screen)WeightApprox. 49 g (without strap)Body MaterialMicrocrystalline zirconium ceramic,polycarbonate, stainless steelStrap TypeSilicone strapStrap Length120 mm (long), 75 mm (short)Strap Width22 mm Display1.39" AMOLEDResolution454 x 454Touch ScreenCorning Gorilla third-generationtempered glass + anti-fingerprint coating PlatformQualcomm Snapdragon Wear 2500ProcessorQuad-core 1.1 GHzMemory512 MB LPDDR3Storage4G eMMC (approx. 1GB of available space)Network TypeTDD-LTE / FDD-LTEWCDMAGSM SensorsBioTrackerTM PPG optical sensorAcceleration sensorGeomagne sensorAmbient light sensorConnectivityBluetooth 4.2 + BLE,Wi-Fi 2.4 GHz 802.11b/g/nMultiple PositioningGPS + GLONASS + GALILEO + WiFiAudioSpeaker, microphoneBattery420mAh lithium-ion polymer battery(typical value)Charging TimeApprox. 1.5 hoursCharging Methodmagnetic / 4-pins POGO pinBattery LifeApprox. 28 hours1Water & Dust Proof GradeIP68Supported DevicesAndroid 5.0 or iOS10.0 and aboveAccessoriesDedicated USB charging baseAppZepp App (formerly known as the Amazfit App) Source: https://www.amazfit.com/en/nexo

The watch has 4G. The watch has an E-Sim. Unfortunately, the watch does not have an ECG.

sim card only works via e-sim which you can register from the app, unfortunately i didn't find out about the belt. why don't you ask digitec or the manufacturer?