Where Are Sessions Stored?
Sessions are stored in a single folder on the computer's filesystem. The first time Ardour is run, it will ask for the default location for this folder, with the initial choice being the current user's home folder.
After the first-run dialog, the default location can still be changed at any time via Edit > Preferences > Misc > Session Management. A particular (different) location for a session can also be specified when creating it, in the New Session dialog.