Technically speaking, Windows 11 does not encrypt your entire system disk, which is divided into different logical volumes. Instead, it encrypts the C: drive, which is the volume that contains Windows and other system files. (This drive is often referred to as the system disk.) Any other volumes on this disk will not be encrypted (nor visible normally while using Windows 11).
If you sign in to Windows 11 with a local account, which we do not recommend, device encryption will not be enabled automatically. This is only one of many reasons why using a Microsoft account is more secure.