Saving and autosave
Nothing is written to your account until you save. Circuit → Save circuit (or ⌘S / Ctrl+S) writes just the circuit you're editing; Project → Save writes the whole project, and Project → Save As saves a copy under a new name or folder, leaving the original untouched. An asterisk next to the project name in the breadcrumb means there are unsaved changes. Autosave, off by default, is a separate setting in Preferences: turn it on and it silently re-saves the project on a timer between 1 and 30 minutes — but only once you've saved it by name at least once, and only when there's something new to write.
Save vs. Save As
Save (⌘S / Ctrl+S) writes the current project to the same place it was last saved; the very first time, since there's no place yet, it behaves like Save As and asks for a name and folder. Save As always asks, letting you branch a copy off under a different name without touching the original.
Reading the asterisk
An asterisk next to the project name in the breadcrumb marks unsaved changes; it disappears the moment a save succeeds. Try to close or reload the browser tab while it's showing and the browser itself warns you first, since anything not yet saved would be lost.
Turning on autosave safely
Enable Autosave in Preferences and pick an interval; the editor then re-saves the project on that timer, but only for a project you've already named and only while there's something unsaved — it never interrupts you with a naming prompt. Autosave is a safety net, not a substitute for a deliberate save: if you're about to close the tab right after a big change, still press Save yourself rather than trust the next tick.