Yes, PPR is a directory in an existion partition. One might want to create a dedicated partition for it too, for example to get proper linux permissions or to get readability in the base (ntfs support is in extensions)
- tcz saves ram because it's mounted, yes
- files in tcz are symlinked to the base, by default not overwriting existing files. Extensions very rarely have need to overwrite files in base, but there is a way to do so with tcz too.
- tce is extracted to root, it overwrites anything
- why would one load several versions of the same package? As usual, we try to have the latest also the best