This is an SSH server running as root using the great Magisk systemless root suite. It includes binaries for arm, arm64, x86, x86_64. However, only arm64 has been tested at all. It requires Android API version 24 or higher (Android 7.0 Nougat and higher).
Download the zip file and install it via the Magisk Manager app. Once this module is available in the Magisk module repository, you can just install it from there.
SSH keys can be put into /data/ssh/root/.ssh/authorized_keys and /data/ssh/shell/.ssh/authorized_keys using your favorite method of editing files.
Note that this file must be owned by the respective user and should have 600 permissions (owner: rw, everyone else: nothing).
The sshd configuration file in /data/ssh/sshd_config can be edited as well, but please be aware that some features usually present in an OpenSSH installation may be missing. Most importantly, password login is not possible using this package.
The ssh daemon automatically starts on device boot. If this is undesired, you can create a file /data/ssh/no-autostart. It will not start the service then.
Once you have written a valid SSH public key into an authorized_keys file (see section 'Configuration' above), you can connect to the device using ssh shell@<device_ip> (unprivileged access) or ssh root@<device_ip> (privileged access), while supplying the correct private key. You will drop into a shell on the device. sftp and rsync should work as usual.
If you want to manually start/stop the sshd-service, you may do so using /data/adb/modules/ssh/opensshd.init start and /data/adb/modules/ssh/opensshd.init stop. This is usually not necessary but may be useful if you use the no-autostart file described earlier.
Note that the opensshd.init script may be in a different place on your device. Magisk explicitly does not give any guarantees about the install location and is free to change it.
Uninstalling the module via the Magisk Manager should also delete the /data/ssh directory.
This contains the host keys for the SSH server and the home directories for the SSH users.
Thus, uninstalling via the Manager should get rid of all traces of this module.
If you wish to keep the runtime data for a later reinstallation of the module, create a file /data/ssh/KEEP_ON_UNINSTALL and the uninstaller will skip this step.
Please don't file Pull Requests against the module repository. The module building is an automated process and will overwrite any changes to the files in the module repository. Feel free to create a Merge Request against the source repository, instead.