The Raspberry Pi Pico comes with 2 MByte FLASH. If more is needed for example to store data or for a data logger, this article shows how it can be extended with a 16 MByte serial flash using the LittleFS file system.
Get Started with Raspberry Pi Pico and C/C++ - OKdo
Get Started with MicroPython on Raspberry Pi Pico
Raspberry Pi Pico (c-sdk)] Storage: Ep2. Multi external flash
GitHub - lurk101/pico-littlefs: A small C/C++ Posix-like journalng
Add extra Storage to the Raspberry Pi Pico with W25Q128 and
Add extra Storage to the Raspberry Pi Pico with W25Q128 and
Getting Started Tutorial With Raspberry Pi Pico W Using
Overview, Adafruit SPI FLASH Breakouts
Add extra Storage to the Raspberry Pi Pico with W25Q128 and