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Linux is a multi-user operating system. Different users and groups of users can be assigned permissions to handle certain files or run certain commands.
root is a special user on Linux. root is a super-user account that can see all files and has permissions to run all commands. Normally this account should only be used for administrative tasks.
root is automatically created when a Linux system is installed.
It's best practice to set up secondary user accounts and give those user accounts required permissions to perform their tasks. https://docs.rackspace.com/support/how-to/linux-server-security-best-practices/
The Linux file system is organised into files and directories. Directories are basically like folders.
Linux has standard directories for typical uses:
https://www.howtogeek.com/117435/htg-explains-the-linux-directory-structure-explained/
https://www.geeksforgeeks.org/linux-directory-structure/