Android
ARCore
Documentation for ARCore in Unity
Tutorial
Introduction to ARCore using Sceneform
iOS
ARKit
ARKIT TUTORIALS!:
Solar System: https://www.udemy.com/ar-solarsystem/
Basic ARKit Drag And Drop: https://github.com/zweg25/ARExample
Object Tracking with Vision APIs: https://developer.apple.com/videos/play/wwdc2018/716/
WebAR
AR.js
aframe-ar (A-Frame support for WebXR; related -- aframe-xr)
HoloLens
Intro tutorials
Mixed Reality Toolkit