A basic configuration is generated at container creation time with the default's recommended for the chosen template as well as extra default keys coming from the default.conf file.
That default.conf file is either located at /etc/lxc/default.conf or for unprivileged containers at ~/.config/lxc/default.conf.
Details about the syntax of this file can be found in: lxc.container.conf(5)
This configuration file is used to set values such as default lookup paths and storage backend settings for LXC.
Details about the syntax of this file can be found in: lxc.system.conf(5)