QNAP NAS internal storage size for booting Debian

I've just downloaded version 4.3.6 of QNAP QTS operating system. It seems to contain amd64 Linux kernel 4.2.8, initrd and root filesystem, glibc 2.21, Apache 2.4.37, PHP 5.6.38, no Perl, Python 2.7.17rc1, BusyBox 1.24.1, rsync 3.0.7, Bash 3.2.57, tar 1.22, etc., (some of it) compiled with GCC 4.9.2. Size of the downloaded firmware file is 208 MiB, after full recursive decompression it's 582 MiB, and after compression with 7z it's 148 MiB. (The kernel is 6.88 MiB, the initrd is 15.69 MiB, both compressed.) Thus to be able to run the QTS operating system, the QNAP device needs storage of at least 582 MiB. My questions: 1. Which storage device contains the boot and root filesystems on a QNAP NAS such as TS-251+ when it is running the QTS operating system? Is it one of the SATA HDDs or is there some other internal storage such as a flash (or SSD)? (Please note that I'm interested only in devices which are at least 582 MiB in size, thus a smaller flash containing the bootloader only is not what I'm looking for.) 2. If I want to install Debian (running natively as a replacement of QTS) to the QNAP NAS TS-251+, can I install it to an internal storage device, and will it boot and run if the NAS contains 0 SATA devices? How much internal storage space will be available for the ext4 boot+root filesystem? 3. What options do I have to boot Debian from? I guess these: internal storage (if available), SATA HDD or SSD, external USB storage. Are there any other options, such as an extra SD card or non-SATA SSD?