Currently it's available on Nothing Phone (2a) , Phone (2) & Phone (2a) Plus (beta)
Ensure your device is on the latest NOS version 3.0, the build number should be:
Phone (2): Pong-V3.0-241207-0124
Phone (2a): Pacman-V3.0-241210-2057
Phone (2a) Plus: PacmanPro-V3.0-241126-1448
To check your current Build number
Go to settings > About Phone > tap on the picture of Phone > Build Number
To enable Circle to search
Go to settings>Special features > Gesture > navigation mode > Circle to search
If you aren’t able to find or enable the Circle to Search function, try rebooting your device