. . . the differences between FAT32, exFAT, NTFS and EXT4 . . . how defragmentation acts . . .
The easiest plain language source on these that I have found is wiki.
As for an explanation of de-fragmentation, that may take some more doing.
Defraging comes to us from the old Winchester, MFM, and RLL drives of the ancient times. (Current events for me!) ;)
In those days you had to set up hdd's manually. IDE (Integrated Drive Electronics) changed all of that, but some of the old tools seem to have hung around.
One of the things you had to do when low level formatting an old drive was set the drive interleave. This was to match the speed of the spindle with the speed of the read/write head.
This would give you the fastest read/write time on that particular drive. Every make and model was different.
Fragmenting occurred when files changed size on a regular basis. Take database, as an example. Most database files grow, and grow, and grow. The original file does not take long to exceed the size of the sector(s) was first written to. So, the file is split, into several parts, all located in different parts of the hdd.
Those parts do not necessarily mean that the file is "fragmented." As long as the parts are sequential, then there should be no noticeable difference in the speed at which the file is read into memory.
Now, as the hdd gets used, filled up, old files deleted, new files added, the free sectors get scattered all over the disk. The file system keeps track of which are the oldest unused sectors, and tries to use those up first. (That is why you can use things like un-delete and shadow copy to recover old files!)
The result is that the spots on the disk where the large files are stored are no longer sequential. The heads have to do more work to get the file loaded into memory. So, now the file takes longer to load.
That is what made correct drive geometry, especially interleave, important. It reduced the seek time, because the physical arm had to do less movement.
On older drives, where the interleave was wrong (more common than you would think,) defragging had to be done often.
As drives got larger, systems improved, and interleave became automatic (IDE did that,) defrag became more and more redundant. In most cases, with modern SATA drives, you should never have to defrag a drive. Some techs will tell you to do it, but I have never found any appreciable increase in read/write speed on the newer drives, after defragging. I imagine it would be a complete waste of time on an SSD drive.
BTW, the fastest way to defrag a drive. Copy the files to a backup drive. Format the original, then copy the files back. All the files will then be written sequentially.