A drawing is saved on your device first. Account sync, folders and exports are how you keep a second copy.
On the device
By default a drawing is saved on the device. It opens fast and works with no internet. Deleting the app deletes local drawings, so export first. Device backups to iCloud only help if you turned them on in iOS.
Account sync and folders
Settings, App, Storage chooses where drawings live and shows sync. Signed-in accounts can sync personal drawings; Free includes 500 MB of account sync, and photos and local drawings have no fixed cap beyond your device. You can also choose the app's iCloud container or any folder from an iOS file provider such as Dropbox or Google Drive. Folders are separate from account sync. Web account sync is live now. The installed apps need an update for it. Check which build you have before you rely on it.
Drawings synced to a team sync live. See Sharing, sync and working together.
Back up and recover
Export a drawing from the Drawings list. The file contains every layout of any type, photos, tags, notes, textures and details. Import it from the Drawings list, from the Files app or an email attachment (Open In Measure Tech PRO), or from the Share Sheet. Export finished drawings, keep a recent copy, and try an import on another device now and then.
A deleted drawing stays in the Recycle bin for 30 days.