Python (packages)

I have a question
How do I know that, for example, what are the functions in the requests library in Python?
And what do those functions do?

Documentation of module/library. Or in repl/interactive console dir and help functions to find attributes/functions of module/library and what they do.

For example:

>>> import math
>>> dir(math)
['__doc__', '__loader__', '__name__', '__package__', '__spec__', 'acos', 'acosh', 'asin', 'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'comb', 'copysign', 'cos', 'cosh', 'degrees', 'dist', 'e', 'erf', 'erfc', 'exp', 'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'gamma', 'gcd', 'hypot', 'inf', 'isclose', 'isfinite', 'isinf', 'isnan', 'isqrt', 'ldexp', 'lgamma', 'log', 'log10', 'log1p', 'log2', 'modf', 'nan', 'perm', 'pi', 'pow', 'prod', 'radians', 'remainder', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'tau', 'trunc']
>>> help(math.pow)
Help on built-in function pow in module math:

pow(x, y, /)
    Return x**y (x to the power of y).

Apart from the in-python options sanity showed, you can also go to the official site of the libraries and read their documentations.
For example, if you wanna know what scikit-learn or pygame can do. And then you might want to watch out for tutorials on there, youtube or whatnot, because some libraries can cover massive amounts of stuff.