How to drive up to millions of ws2812 or APA104 LEDs?
A high-definition (1080p30) video stream represents about 1.5 Gb/s of data.

Commercial video walls (a.k.a. "Jumbotron") are built up from edge-stackable modules (anywhere from 1000 to 10,000 LEDs per module) that are typically networked together using Ethernet. A master controller receives a video stream over any standard connection (HDMI, etc.), and then formats the data to send to the individual modules using a proprietary protocol.

Obviously, there are many ways to produce HDMI video from computers, disk players, etc.