You are somewhat correct that the tmpfs results you are seeing in the output of df -h "a sort of RAM disk", but there's much more to these locations than meets the eye. Allow me to explain why they exist and whether they can be removed or not.
Each of these locations are actually API file systems and are critical to the operation of a systemd-based system. systemd mounts these locations during system initialisation literally before it does anything else.
Let's look at each of these, and why they exist:
/dev/shm
This exists pretty much only on Debian-based operating systems. /dev/shm is actually /run/shm on other distributions such as Fedora and SuSE. This tmpfs has the specific purpose of implementing POSIX shared memory on your operating system. Removing it will cause applications that use POSIX shared memory to fail.
This would be a bad thing.
/sys/fs/cgroup
This is where systemd and other daemons mount the various control group hierarchies that are available on your operating system. Removing it will stop the parts of your operating system, that depend from control groups, working.
This, too, would be a bad thing.
/tmp
As the name suggests, this is where applications expect to be able to put short-stay temporary files. Removing it will make a fair number of tools that use temporary files rather crashy.
Naturally, this would be a bad thing.
/run
This used to be /var/run and is where specialised types of temporary files that can potentially last until the next system bootstrap are placed. In /var/run you will find your PID files, the systemd non-persistent journal, UNIX-domain sockets, FIFOs, and other similar things. Removing it will make the systemd journal grow error logs as fast as your storage device can record them; break the vast majority of daemons that still, even today, use PID files; and wreck udev, the systemd "multi-seat" system, plus a whole bunch of other subsystems.
Do not do this unless you wish to see just how ungracefully an operating system can fail.
/dev
This is generally where you will find all of the device files for character and block devices are stored on Linux. A lot of programs and subsystems expect conventional names like /dev/tty, /dev/null, /dev/zero, /dev/console, /dev/fd/0, /dev/sda, and so forth to work. Removing this will break so much of the system that it would become effectively useless. This is a devtmpfs rather than a tmpfs, the difference being that the former is automatically populated and unpopulated with device file entries by the kernel itself, as devices are loaded/enabled and disabled/unloaded in the kernel.
While each of these locations do consume additional memory, the system is generally smart enough to know when these should exist in RAM and when they can be shuffled off to swap. Unless you are running a system with less than 768MB of total memory (RAM plus all swap files and — optionally — partitions), you should be fine.
I hope this answers your questions.
