https://www.flatpanelshd.com/flatforums/viewtopic.php?f=31&t=12779
Apple TV+ has highest the streaming quality we've seen for 4K so far. It's higher than most iTunes movies. Over the weekend, I took the first batch of measurements. I'm planning to do more later. It's worth pointing out that Apple TV+ uses variable bitrate with a quite wide span from low to top, so the console must run for some time to calculate the average bitrate.
See has the highest bitrate so far. 29 Mb/s average video bitrate and 41 Mb/s video peak (I don't know if it hits that occasionally or rarely, since the console only reports average and peak).
So that's approximately 1.5-2x the video bitrate of a typical HD Blu-ray disc (due to HEVC vs. AVC) and around half of a typical UHD Blu-ray disc (also using HEVC).