Benjamin Mayo, 9to5Mac:

With iOS 14, Apple has added a new automatic switching feature to the AirPods and AirPods Pro earbuds. This feature is meant as a convenience; when you start playing content on another Apple device, the AirPods connection automatically switches in unison. However, some people don’t like this behavior and would rather connect to each device manually, just like how it worked in iOS 13 and earlier. Good news! There is a way to get the old behaviour back.

First things first, I’ve found the behavior just a bit buggy. I’ve been listening to content on my Apple TV through my AirPods Pro, then fired up my iPad, which brought up the automatic switch message and showed the AirPods icon on my iPad, as if the switch had happened. But the Apple TV content still played on my AirPods. Odd, but early days still, I’m sure that will get resolved.

As to the linked post, worth reading, especially the last two paragraphs which help clarify some confusing setting wording, specifically the phrase “‘When Last Connected to This iPhone”.