The DF command and deciphering it [duplicate]

Question

I've been using Linux for about 4 weeks now, more specifically Ubuntu. I wanted to find the remaining disk space I have left and was told that the df command will give me all the information I need to answer that question but there is quite a lot of information there. I was just looking for a simple response such as 225gb out of 500gb used etc but instead I get a lot more information.

    Filesystem      Size  Used Avail Use% Mounted on
udev            3.9G     0  3.9G   0% /dev
tmpfs           788M  1.9M  786M   1% /run
/dev/sda5       104G   15G   84G  15% /
tmpfs           3.9G     0  3.9G   0% /dev/shm
tmpfs           5.0M  4.0K  5.0M   1% /run/lock
tmpfs           3.9G     0  3.9G   0% /sys/fs/cgroup
/dev/loop0       56M   56M     0 100% /snap/core18/1988
/dev/loop2      256M  256M     0 100% /snap/gnome-3-34-1804/36
/dev/loop3      219M  219M     0 100% /snap/gnome-3-34-1804/66
/dev/loop1       56M   56M     0 100% /snap/core18/1944
/dev/loop4       65M   65M     0 100% /snap/gtk-common-themes/1514
/dev/loop6       32M   32M     0 100% /snap/snapd/11036
/dev/loop5       50M   50M     0 100% /snap/snap-store/467
/dev/loop7       63M   63M     0 100% /snap/gtk-common-themes/1506
/dev/loop8       33M   33M     0 100% /snap/snapd/11107
/dev/loop9       52M   52M     0 100% /snap/snap-store/518
/dev/sda2        96M   33M   64M  35% /boot/efi
tmpfs           788M   56K  788M   1% /run/user/1000

Above is the output to df -h, so a few questions.

Why is there so many different options showing such as udev,tmpfs, dev/loop9 etc, what are these?

I am guessing that /dev/sda5 is my Linux partition, the only reason I determined this was because it has the most space used and most space available, is everybody's Linux partition (main partition where data is stored) named /dev/sda5?

and why is there another /dev/sda in the name /dev/sda2?

I'm also quite unsure what the Use% Mounted On column is for, why is there folders listed in this column?

Thanks