Ok i solved my issue.
Doing my researches I found a library that is used with Arduino33 that mounts the same chip.
The library is FS_Nano33BLE
After installation you will find an Example called “FS_Test”, if you try to compile it you will get an error saying that it’s supposed to work only with nRF52840 platform… even if indeed it is… well
Just remove that check from the library and problem solved. It works. 
Follow this comment: Logging Data to Onboard Flash Memory of Seeed XIAO BLE Sense - Stack Overflow to do it
On macOS the file is located in: /Users//Documents/Arduino/libraries/FS_Nano33BLE/src/FS_Nano33BLE.h