can is entirely local. Your notes are stored on your machine, wherever you've set up your writing folder. You can use tools like
git to sync your files across machines (which should work really well), but
can core tools should never access your network. It is possible that some tools will be built to aid syncing / working in a distributed manner - but those will be explicit and entirely opt-in if and when they exist at all.
On top of this,
can stores its notebooks in a plaintext format so you can always get your notes back.