You don't mention what you are looking for in terms of a "running configuration file". In Unix/Linux there are many of these. Most of them can be found in the /etc directory or its subdirectories but not all of them are located in /etc.
On NVRAM.
Boot Configuration Data (BCD)
NVRAM is that router component in which start up configuration file is stored.
NVRAM
It will boot into ROMMON mode. It will ignore the start-up configuration file. It will look for the start-up configuration file on the TFTP server. It will attempt to load the start-up configuration file that is stored in NVRAM.
The running configuration on a router or switch is stored in the device's RAM (Random Access Memory) while it is running. It's a temporary configuration that is lost when the device is powered off or restarted. To view the running configuration, you can use the "show running-config" command in the device's CLI (Command Line Interface).
A technician would enter the command copy startup-config running-config to copy the configuration stored in the startup configuration file (which is saved in NVRAM) to the running configuration (stored in RAM). This action allows the device to apply previously saved settings without needing to re-enter them manually. It is often used after making changes to ensure that the system is configured as intended. Additionally, it can help restore configurations after a reboot or power cycle.
wr or copy running-config startup config
The running configuration file on S1 will be saved via TFTP to a file named tokyo-config.
The proc file system, commonly referred to as /proc, is a virtual filesystem in Linux that provides an interface to kernel data structures. It is not stored on a physical disk but is generated dynamically in memory by the kernel when the system is running. The contents of /proc reflect the current state of the kernel and processes, allowing users and applications to access system information and configuration.
SDRAM
The Motherboard configuration is stored in the BIOS setup