[Sony Xperia XZ1 Compact ROMs, Kernels, Recoveries,] j4nn:
In my opinion the AT_FDCWD is not used for any special purpose, it is just the openat() system call which is used as "open" and it happens to have this parameter.

Please check your block device nodes setup - for example the 'xbl' points in your listing to 'vda', but it should be 'vda1' in my opinion.

Here listing from xz1c:

lilac:/ # ls -l /dev/block/bootdevice/by-name/
total 0
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 FOTAKernel -> /dev/block/sda47
lrwxrwxrwx 1 root root 15 1971-02-23 05:52 LTALabel -> /dev/block/sda2
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 Qnovo -> /dev/block/sda60
lrwxrwxrwx 1 root root 15 1971-02-23 05:52 TA -> /dev/block/sda1
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 abl -> /dev/block/sda11
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 ablbak -> /dev/block/sda12
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 apdp -> /dev/block/sda40
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 appslog -> /dev/block/sda58
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 bluetooth -> /dev/block/sda39
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 boot -> /dev/block/sda38
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 cache -> /dev/block/sda53
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 cmnlib -> /dev/block/sda16
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 cmnlib64 -> /dev/block/sda18
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 cmnlib64bak -> /dev/block/sda19
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 cmnlibbak -> /dev/block/sda17
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 ddr -> /dev/block/sda48
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 devcfg -> /dev/block/sda21
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 devcfgbak -> /dev/block/sda22
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 devinfo -> /dev/block/sda13
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 diag -> /dev/block/sda59
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 dpo -> /dev/block/sda42
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 dsp -> /dev/block/sda37
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 frp -> /dev/block/sda56
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 fsc -> /dev/block/sda51
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 fsg -> /dev/block/sda34
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 fsmetadata -> /dev/block/sda61
lrwxrwxrwx 1 root root 15 1971-02-23 05:52 hyp -> /dev/block/sda7
lrwxrwxrwx 1 root root 15 1971-02-23 05:52 hypbak -> /dev/block/sda8
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 keymaster -> /dev/block/sda14
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 keymasterbak -> /dev/block/sda15
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 keystore -> /dev/block/sda55
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 limits -> /dev/block/sda44
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 logfs -> /dev/block/sda46
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 misc -> /dev/block/sda54
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 modem -> /dev/block/sda36
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 modemst1 -> /dev/block/sda49
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 modemst2 -> /dev/block/sda50
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 msadp -> /dev/block/sda41
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 oem -> /dev/block/sda63
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 persist -> /dev/block/sda33
lrwxrwxrwx 1 root root 15 1971-02-23 05:52 pmic -> /dev/block/sda9
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 pmicbak -> /dev/block/sda10
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 rdimage -> /dev/block/sda62
lrwxrwxrwx 1 root root 15 1971-02-23 05:52 rpm -> /dev/block/sda3
lrwxrwxrwx 1 root root 15 1971-02-23 05:52 rpmbak -> /dev/block/sda4
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 sec -> /dev/block/sda35
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 splash -> /dev/block/sda43
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 ssd -> /dev/block/sda52
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 sti -> /dev/block/sda20
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 storsec -> /dev/block/sda23
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 storsecbak -> /dev/block/sda24
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 system -> /dev/block/sda65
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 toolsfv -> /dev/block/sda45
lrwxrwxrwx 1 root root 15 1971-02-23 05:52 tz -> /dev/block/sda5
lrwxrwxrwx 1 root root 15 1971-02-23 05:52 tzbak -> /dev/block/sda6
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 tzxfl -> /dev/block/sda29
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 tzxflattest -> /dev/block/sda27
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 tzxflattestbak -> /dev/block/sda28
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 tzxflbak -> /dev/block/sda30
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 userdata -> /dev/block/sda66
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 vbmeta -> /dev/block/sda57
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 vendor -> /dev/block/sda64
lrwxrwxrwx 1 root root 15 1971-02-23 05:52 xbl -> /dev/block/sdb1
lrwxrwxrwx 1 root root 15 1971-02-23 05:52 xblbak -> /dev/block/sdc1
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 xfl -> /dev/block/sda25
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 xflbak -> /dev/block/sda26
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 xflkeystore -> /dev/block/sda31
lrwxrwxrwx 1 root root 16 1971-02-23 05:52 xflkeystorebak -> /dev/block/sda32
lilac:/ #

My strace as run the fota-ua shows this corresponding to yours:

readlinkat(AT_FDCWD, "/dev/block/bootdevice/by-name/system", "/dev/block/sda65", 4095) = 16
openat(AT_FDCWD, "/dev/block/sda", O_RDWR|O_SYNC) = 8
ioctl(8, BLKGETSIZE64, [31989956608]) = 0
readlinkat(AT_FDCWD, "/dev/block/bootdevice/by-name/xbl", "/dev/block/sdb1", 4095) = 15
openat(AT_FDCWD, "/dev/block/sdb", O_RDWR) = 9
fstat(9, {st_mode=S_IFBLK|0600, st_rdev=makedev(8, 16), ...}) = 0
ioctl(9, BLKGETSIZE64, [4194304]) = 0
...
readlinkat(AT_FDCWD, "/dev/block/bootdevice/by-name/TA", "/dev/block/sda1", 4095) = 15
openat(AT_FDCWD, "/dev/block/sda", O_RDWR) = 9

Note that the fota-ua reads a partition's symlink but it then opens the whole block device, not the particular partition device.

It probably walks the partition table on it's own, not relying on linux.