This free buzz is a practice technique is popular among a lot of top brass players. It is widely championed by legendary hornist and pedagogue, Julie Landsman. I have incorporated her thoughts and perspective into my own teaching and have developed similar exercises to share with my own students. I find them highly effective in encouraging regular, consistent airflow as well as producing a simple, yet powerful sound.
You can find Professor Landsman’s Free Buzz Method here: Carmine Caruso Method
I suggest buzz routine letter B.
David Cooper, newly minted principal horn of the Chicago Symphony Orchestra, has put out a popular YouTube video of his daily warmup and routine. You can see his free buzzing approach by clicking here.