sorry, confusing, two different roots, nothing to do with each other.

— the username root, which is the user you login as over ssh.
for user root, the homedir is /root

root@GL-AR750S:/# echo $HOME
/root
root@GL-AR750S:/# cd ~
root@GL-AR750S:~# pwd
/root

— the root of the router’s file system /
on windows, the equivalent is c:\

root@GL-AR750S:/# ls -al
drwxr-xr-x    1 root     root           608 Jul 13 16:11 .
drwxr-xr-x    1 root     root           608 Jul 13 16:11 ..
drwxr-xr-x    2 root     root           792 Apr  7 04:42 bin
drwxr-xr-x    5 root     root          1340 Jul 14 15:37 dev
drwxrwxr-x    1 root     root          2424 Jul 29  2021 etc
drwxr-xr-x    1 root     root           520 Jul 29  2021 lib
drwxr-xr-x    1 root     root           224 Jul 12 17:37 mnt
drwxr-xr-x    4 root     root           360 Apr  7 04:43 overlay
dr-xr-xr-x   83 root     root             0 Dec 31  1969 proc
drwxrwxr-x   16 root     root           235 Apr  7 04:42 rom
drwxr-xr-x    1 root     root           624 Jul 13 16:48 root
drwxr-xr-x    1 root     root           232 Jul 12 10:38 sbin
dr-xr-xr-x   13 root     root             0 Dec 31  1969 sys
drwxrwxrwt   23 root     root           700 Jul 14 17:20 tmp
drwxr-xr-x    1 root     root           480 Jul 29  2021 usr
lrwxrwxrwx    1 root     root             3 Apr  7 04:42 var -> tmp
drwxr-xr-x    1 root     root           232 Jul 29  2021 ww