Photos: Choose a different Library
Pressing the option key and double clicking the photos app, does not open the "Choose a Library" dialog. It opens the last library I used.
iMac 27″, macOS 11.6
On macOS 11 Big Sur quit Photos before you try to switch to a different library.
Then hold down the options key ⌥ while double-clicking Photos and keep holding down the options key firmly, until the library chooser dialog appears.
Or quit Photos and then double-click the Photos Library you want to open directly.