This application note explains the technical details of how 256K user firmware binaries work on Gen 3 devices in Device OS 3.1 and later.
You probably don’t need to worry about the technical details about how the user firmware binary is moved while retaining forward and backward binary compatibility, but if you’re curious, it’s in there.
https://docs.particle.io/datasheets/app-notes/an033-256K-user-binaries/