Where are backup files stored on my Mac?

By default, 2Do for Mac stores backups inside its sandboxed container:

~/Library/Containers/com.guidedways.TodoMac/Data/Library/Application Support/Backups

The fastest way to open the folder is via Terminal:

open ~/Library/Containers/com.guidedways.TodoMac/Data/Library/Application\ Support/Backups

Use 2Do > Settings > Backups > Manage Backups > Save as... when you want to save a backup somewhere easier to move, such as iCloud Drive, Dropbox, or an external drive. See Backups (Mac) for the full guide.