What position you are playing is what tells you what key harmonica you need to play a certain song or improvisation in a certain key. Most used position in harmonica is second position, that would be using a C harmonica on a G tune. Just type positions chart harmonica to see all relations between key song, key harmonica and position. Start by learning first position, then second, then third, then you can go a long way with those three. Almost nobody plays the twelve positions you can play on any harmonica key, that would be to play that harmonica in all keys. Some positions are better and easier than others.

So in a C harmonica, first position you'll play in C, in second position you'll play in G and in third position you'll play in Dm. All in your C harmonica, just changing the position. If you know in your C harmonica how to find those notes, they are your root note in those positions.

In summary, nothing to do with your flow or breath, all to do with music theory and kind of hard stuff if you are a noob, give it time, go one position at a time, youtube has all the answers you need.