The Librem 5 is a phone running a Debian derivative named PureOS that is designed for all open software and hardware. It is manufactured by Purism.
The phone comes with a terminal emulator built in, you can run "sudo bash" to get root. Then after changing the password for the "purism" account (the default account for GUI tasks) you can run "apt install openssh-server" to enable remote logins. After that it can be run just like any other Linux system.
The kernel parameters are signed and if they are changed the boot process will fail. If that happens there is no convenient way of recovering apart from reflashing. The images for reflashing are built with recent security updates so after a reflash all the latest security fixes should be applied.