len is a built-in function: https://docs.python.org/3/library/functions.html#len

When you do len("something") you are passing the string something to it, and it returns how long it is. You can pass it other things like lists or sets, and it will tell you how many things are in them, too.

>>> len
<built-in function len>
>>> len("something")
9

>>> len([1,2,3,4])
4
>>> len({"a", "b", "c"})
3

If you were to try to do "something".len() it would try to call the function "len" that exists on str. There isn't one.

>>> "something".len()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'len'

https://docs.python.org/3/library/stdtypes.html#textseq

Scroll down a little to "String Methods" and you can see what methods are available on strings.

This is kind of language specific. Now you know that when you want to know how long something is in Python, you generally use the built-in len. If you're dealing with some other type of object, you'd check what methods it provides and what it inherits from. There's a lot of documentation reading in software development. A good IDE also helps.