See documentation here.
Cloudless stores tasks in a todo.txt file on the device. A separate app (such as Syncthing) can be used to Sync the file.
Because the todo.txt file could be anywhere on the device to allow sync by a different app, the Cloudless version requests full storage access. This is unfortunately the only way to make common use case work while staying sane.
Nextcloud stores tasks in a todo.txt file using Nextcloud.
Nextcloud stores tasks in a todo.txt file on any WebDAV server.
The Dropbox version is not available from F-Droid. You can download builds of the different versions from: https://mpcjanssen.nl/artifacts/