I am using debian 9.4, the kernel version shown by 'uname -a' is different, what's the actual kernel version? What does '4.9.82-1+deb9u3' mean?

'apt-get source linux' can get the source code of current kernel, then I can get original kernel tar file and debian patch.

'linux_4.9.82-1+deb9u3.debian.tar.xz' contains many files, what are them used for? how to patch the it to 'linux_4.9.82.orig.tar.xz ' ?

'linux-4.9.82' is the kernel source folder outputted by 'apt-get source' command, which can be compiled to vmlinux. However, I usually can't debug the crash dump files with the vmlinux, as 'crash' command gives the error message: mismatch version. Should I download 'kernel 4.9.0' from kernel.org instead of 'linux-4.9.82' to debug my current running kernel ?

several linux kernels can be installed at the same time, but the system only boots into one.
'uname' will only reprt on the currently running kernel.
please read
to understand all the info it gives.

Yes, I have read man document. 'uname -v' shows kernel version, while 'uname -r' shows kernel release. I can't understand what's the difference.
The OS is newly installed, so it can't exists multi kernel now.

to be honest, i don't fully understand that bit either, but it seems that one refers to your distro's packaged kernel, and the other to the kernel version itself.

correct.

The files inside the kernel source package are mostly module source code. If you build a kernel, use the source package from the debian repos. The vanilla kernel is missing the debian patches.