Where does language knowledge come from? Intelligence, innate language ideas, behaviour?

You say the world is not flat. How do you know that this is true? You say you know what a perfect circle is. Did someone tell you? How do you know that what they told you is true? Can your measurement be perfect? You say God exists or does not exist. How do you know this? Do ideas exist? Do we have minds and are there ideas in our minds? We have knowledge but what is it and where did it come from?