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