Which file from the /proc file system contains a list of all currently mounted devices?

The file /proc/mounts contains a list of all currently mounted devices in the system. It is a pseudo-file that is dynamically generated by the kernel and reflects the actual state of the mount table. It has a similar format to /etc/fstab, but shows the actual mount options and file system types used by the kernel. The file /proc/mounts can be read by any user, but only the root user can modify it. The file /proc/mounts is also known as /proc/self/mounts, which is a symbolic link to the mounts file for the current process. References:

Understanding the /proc/mounts, /etc/mtab and /proc/partitions files

How to get the complete and exact list of mounted filesystems in Linux?

The /proc Filesystem