Why when explaining, giving examples of shell command are people so often providing shortened arguments. It makes it all seam like some random letters you have to remeber by heart. Instead of -x just write --extract. If in the end they endup using the tool so often they need to write it fast they'll check the shortcuts.
I don't even mind the shortened arguments too much, though it doesn't help. It's more that every example seems to smush them together into a string of letters.
I would have found
tar -x -f pics.tar ./pics
to be clearer when I was learning. There's plenty of commands which allow combining flags but every tar tutorial seems to do it from the beginning.
They are random letters you have to learn by hard. No one uses the long form. If someone just needs to use it one time they will copy the line from somewhere.
tar -xf is not really special combining short options isn't uncommon.
Where tar is nonstandard is that you can leave out the -, tar xf is actually how POSIX specifies it. And we've kinda come full circle on that one with many modern utilities using a command syntax, you can read tar xf as "tar extract file" just as you can read git pull as, well, "git pull".
If you want to see a standard command with truly non-standard syntax have a look at dd.
Many do as it's considered good practice, but it's not guaranteed, it just depends on the individual command (program). Usually you can use the --help option to see all the options, so for instance tar --help.
Most commands will have expanded arguments started with 2 dashes that usually look like '--verbose-name-of-option', they're usually listed in the man page/documentation along with the abbreviated letter version