findmnt is a command to display the location(s) of one or more filesystems, either actual or potential.
There are three main ways to invoke this command, plus internal help (much like the man page).
default: show mounted filesystems
findmnt [-k|--kernel] ...
search in /etc/fstab and /etc/fstab.d
findmnt -s|--fstab ...
(be aware that /etc/fstab.d is not well-supported elsewhere; it's more of a wish-list item)
search in /etc/mtab
findmnt -m|--mtab] ...
do something else
Prints help in man page format, and quits.
Selecting filesystems to list and controlling format
Usually, the rest of the command line has to do with selecting filesystems to list (rather than all of them) and controlling the format.
Most (all?) Linux distributions incorporate this from the [util-linux] project.