By default, the root directory for Docker is typically /var/lib/docker.

~]$ sudo ls -l /var/lib/docker
total 24
drwx--x--x.  4 root root   120 Aug 27  2021 buildkit
drwx--x--x.  3 root root    20 Aug 27  2021 containerd
drwx--x---.  7 root root  4096 Oct 16 06:00 containers
drwx------.  3 root root    22 Aug 27  2021 image
drwxr-x---.  3 root root    19 Aug 27  2021 network
drwx--x---. 81 root root 12288 Oct 19 04:44 overlay2
drwx------.  4 root root    32 Aug 27  2021 plugins
drwx------   2 root root     6 Oct 19 04:44 runtimes
drwx------.  2 root root     6 Aug 27  2021 swarm
drwx------   2 root root     6 Oct 19 04:44 tmp
drwx------.  2 root root     6 Aug 27  2021 trust
drwx-----x.  7 root root  4096 Oct 19 04:44 volumes

This can be changed by setting data-root in /etc/docker/daemon.json. If this JSON file does not exist, you can simply create it.

{
  "data-root": "/path/to/directory"
}

And then reload the system daemons.

systemctl daemon-reload

And restart the Docker service.

systemctl restart docker

A bunch of default Docker files and directories will be created in whatever directory you set for data-root. This is actually a great way to test something without impacting your "normal" docker install and then when done testing, simply remove data-root from daemon.json, reload the daemon, restart Docker, remove the temporary directory, and you'll be back to your "normal" Docker install.

~]$ sudo ls -l /tmp/dockertmp
total 4
drwx--x--x+ 4 root root  148 Oct 19 05:39 buildkit
drwx--x--x+ 3 root root   28 Oct 19 05:39 containerd
drwx--x---+ 2 root root   10 Oct 19 05:39 containers
drwx------+ 3 root root   30 Oct 19 05:39 image
drwxr-x---+ 3 root root   27 Oct 19 05:39 network
drwx--x---+ 7 root root 4096 Oct 19 05:43 overlay2
drwx------+ 4 root root   44 Oct 19 05:39 plugins
drwx------+ 2 root root   10 Oct 19 05:39 runtimes
drwx------+ 2 root root   10 Oct 19 05:39 swarm
drwx------+ 2 root root   10 Oct 19 05:44 tmp
drwx------+ 2 root root   10 Oct 19 05:39 trust
drwx-----x+ 2 root root   62 Oct 19 05:39 volumes