the current version of VLC is 3.0.16-2(pacman). What does the -2 stand for in the version as in the website: https://www.videolan.org/ , it only list 3.0.16. Same for many packages.

the -number is the package build number - so the number two designates the second time the package was build. The build can happen for several reasons - often it is required when depending arch system libraries has been updated.