why there are so many tty files in the dev directory? it is confusing that there are so many tty files in the directory...

by the way, what each file in /dev stands for, do they stand for physical devices which are part of my android device? for example, which file stands for the raw storage device in /dev directory?
That's because there are so many devices available. Entries in /dev are not restricted to physical devices, but also include logical ones. You just named one: TTY. You don't really think you've got 63 "connectors" somewhere to attach keyboards etc. to? Also, /dev/random is nothing physical, but rather "software" (a random-number generator provided by the system itself). Same applies to things like /dev/null, /dev/zero, and several others.

For more/deeper details, you might refer to following articles:


Linux Filesystem Hierarchy
Device file (Wikipedia) -- recommended for this topic, as it explicitly included Pseudo-devices
Linux System Administrator's Guide - The / dev directory