Depends on your definition of freedom. Freedom is an ambiguous word limited by the restrictions of language. To Kant freedom is the faculty in reason to give the universal moral laws unto your will which is like in effect, the observer effect making the universal moral laws appear in existence. In doing so, you do not allow objects of the senses to subject your will unto them due to a relative illusory self interest. This is Heteronomy.

Essentially freedom is to detach yourself from the empirical relative physical world. The self, the ego is what enslaves you through your desire and grasping constantly seeking a past sense of happiness which is only a empirical ideal, not an absolute ideal.

Even this freedom would seem limited but it is the extent of freedom that we as rational beings can achieve.

If the law didnt exist than morality as we know it wouldnt exist and we would be naturally inclined towards whichever universal law holds sway in whichever dimension we are apart of. We are moral because it is a universal law, and nature compels us to be moral as an end in itself; Its not out of self interest. Those who are moral out of self interest like politicians are actually not moral in themselves but only appear to be moral.

All a priori ideas are universal coming from a place we cannot conventionally experience by our limited senses. Yet we are able to tap into this intellect(universal laws) through practical-pure reasoning where there is no self interest or sense perception. Hence pure. As rational beings we then make determinant judgements a posteriori or reflective judgements a priori.

A priori always precedes a posteriori otherwise you would live off of mere animal instinct. It is our capacity of practical reasoning we are able to abstract ideas to explain our empirical experiences. By this capacity we also discover laws which cannot be sensed but yet hold universal sway morally on rational beings. I.e The law of contradiction shows if we made lieing a universal law, nature couldn't operate. We can't test this but we know it a priori.

Where these laws and ideas come from, Kant doesn't even know. He says himself he can rationalize the categorical imperatives, but cannot comprehend the cause of them. As for why man is able to give these laws unto himself adopting the maxim of freedom, Kant speculates intelligent design but doesn't postulate anything. He merely lays down the moral laws already there universally conceived only by practical reasoning