[OnePlus 8T ROMs, Kernels, Recoveries, & Other Deve] card13:
Was curious how the Recovery is mounting things so I looked at the /proc/mounts

1|OnePlus8T:/ # cat /proc/mounts

rootfs / rootfs rw,seclabel,size=5680212k,nr_inodes=1420053 0 0

tmpfs /dev tmpfs rw,seclabel,nosuid,relatime,size=5867776k,nr_inodes=1466944,mode=755 0 0

devpts /dev/pts devpts rw,seclabel,relatime,mode=600,ptmxmode=000 0 0

proc /proc proc rw,relatime,gid=3009,hidepid=2 0 0

sysfs /sys sysfs rw,seclabel,relatime 0 0

selinuxfs /sys/fs/selinux selinuxfs rw,relatime 0 0

tmpfs /mnt tmpfs rw,seclabel,nosuid,nodev,noexec,relatime,size=5867776k,nr_inodes=1466944,mode=755,gid=1000 0 0

tmpfs /apex tmpfs rw,seclabel,nosuid,nodev,noexec,relatime,size=5867776k,nr_inodes=1466944,mode=755 0 0

tmpfs /linkerconfig tmpfs rw,seclabel,nosuid,nodev,noexec,relatime,size=5867776k,nr_inodes=1466944,mode=755 0 0

tmpfs /mnt/installer tmpfs rw,seclabel,nosuid,nodev,noexec,relatime,size=5867776k,nr_inodes=1466944,mode=755,gid=1000 0 0

tmpfs /mnt/androidwritable tmpfs rw,seclabel,nosuid,nodev,noexec,relatime,size=5867776k,nr_inodes=1466944,mode=755,gid=1000 0 0

none /acct cgroup rw,nosuid,nodev,noexec,relatime,cpuacct 0 0

tmpfs /tmp tmpfs rw,seclabel,relatime,size=5867776k,nr_inodes=1466944 0 0

tmpfs /storage tmpfs rw,seclabel,relatime,size=5867776k,nr_inodes=1466944,mode=050,gid=1028 0 0

pstore /sys/fs/pstore pstore rw,seclabel,relatime 0 0

none /config configfs rw,relatime 0 0

adb /dev/usb-ffs/adb functionfs rw,relatime 0 0

fastboot /dev/usb-ffs/fastboot functionfs rw,relatime 0 0

^Notice no /system listed

OnePlus8T:/ # cat /proc/mounts

rootfs / rootfs rw,seclabel,size=5680212k,nr_inodes=1420053 0 0

tmpfs /dev tmpfs rw,seclabel,nosuid,relatime,size=5867776k,nr_inodes=1466944,mode=755 0 0

devpts /dev/pts devpts rw,seclabel,relatime,mode=600,ptmxmode=000 0 0

proc /proc proc rw,relatime,gid=3009,hidepid=2 0 0

sysfs /sys sysfs rw,seclabel,relatime 0 0

selinuxfs /sys/fs/selinux selinuxfs rw,relatime 0 0

tmpfs /mnt tmpfs rw,seclabel,nosuid,nodev,noexec,relatime,size=5867776k,nr_inodes=1466944,mode=755,gid=1000 0 0

tmpfs /apex tmpfs rw,seclabel,nosuid,nodev,noexec,relatime,size=5867776k,nr_inodes=1466944,mode=755 0 0

tmpfs /linkerconfig tmpfs rw,seclabel,nosuid,nodev,noexec,relatime,size=5867776k,nr_inodes=1466944,mode=755 0 0

tmpfs /mnt/installer tmpfs rw,seclabel,nosuid,nodev,noexec,relatime,size=5867776k,nr_inodes=1466944,mode=755,gid=1000 0 0

tmpfs /mnt/androidwritable tmpfs rw,seclabel,nosuid,nodev,noexec,relatime,size=5867776k,nr_inodes=1466944,mode=755,gid=1000 0 0

none /acct cgroup rw,nosuid,nodev,noexec,relatime,cpuacct 0 0

tmpfs /tmp tmpfs rw,seclabel,relatime,size=5867776k,nr_inodes=1466944 0 0

tmpfs /storage tmpfs rw,seclabel,relatime,size=5867776k,nr_inodes=1466944,mode=050,gid=1028 0 0

pstore /sys/fs/pstore pstore rw,seclabel,relatime 0 0

none /config configfs rw,relatime 0 0

adb /dev/usb-ffs/adb functionfs rw,relatime 0 0

fastboot /dev/usb-ffs/fastboot functionfs rw,relatime 0 0

/dev/block/dm-0 /mnt/system ext4 ro,seclabel,relatime,discard 0 0

^/system only shows up when you use the menu option "Mount /system" but manually issuing mount doesn't work since it's not defined by default