i need a module for more involved command line parsing

[snippsat]

so what does @my_decorator really do?

It's not like the web are lacking info about decorators if you search
First hit google Primer on Python Decorators.

In my example with @make_bold,if take away all the magic of @.
It's just functions foo get passed to make_bold function.
make_bold return new_func which added stuff to foo function.

def make_bold(fn):
    '''Do not need to see me'''
    def new_func():
        return f"<b>{fn()}</b>"
    return new_func

def foo():
    return 'python-forum'

make_bold = make_bold(foo) # The cooler way is @make_bold 

The same results.

>>> make_bold()
'<b>python-forum</b>'

are decorators always functions? what if some arbitrary function is used?

No,look at link given.
It's also used in Data Classes.

If you really need to look at source before use Click,then it's all open Click GitHub