You can wire up something like an HC-05 module (https://www.ebay.co.uk/itm/125085159709) and then connect to the Pico via Bluetooth UART connection. This is a bunch of info on doing it with the original Espruino board which even had a space on it: http://www.espruino.com/Bluetooth
However I'd strongly suggest that you look at one of the Espruino devices (like MDBT42 Breakout - http://www.espruino.com/MDBT42Q) that has bluetooth already on it - that would be so much easier to use. More compact, and it'd use a lot less power too
Thanks alot for the response!
Thanks for the response. Can you help me with the sample code on how to transfer data from pico to smartphone?
I'm currently working on a project. I need help connecting my Pico to the smartphone module via Bluetooth, Please help me on which bluetooth module to use and if you can provide me any sample code for the Pico part.