Does backup always acquire full backups (all backup targets)?
No. In this menu, unless you change the backup target or change the backup plan, for the first time all the files specified as backup targets are acquired, and only the files that have been changed are acquired after the second time.
On the other hand, at the time of full system restore, it is possible to restore the full system even for the second and subsequent incremental backups by combining with the past backup inside the system.
For example, if you set a daily backup, the daily backups succeeded and within the retention period are displayed as candidates and you can choose from them to perform a full system restore.