Sorry to be a party pooper but those aren't tubas.
The two on the right hand side are euphoniums and the left hand side is a baritone (and likely the same next to her).
They are almost certainly playing harmonies, in fact the two euphoniums (both with their 1st valve down) are probably playing in harmony as opposed to in unison. As for air flow, generally speaking, you will continue with your air flow over an entire phrase and using your tongue to articulate the change in pitch and rhythm. Think of it in terms of singers, they don't stop their air flow for each individual note/word they sing or the phrases would sound disjointed. In summary, for each note change, your fingers need to change valves, lips need to change embouchure and tongue needs to articulate simultaneously.
Below is a link to my favorite Xmas brass band track:
https://www.youtube.com/watch?v=FqhoQzvBgDM