I have a trombone sitting in my house I’ve been wanting to learn as I have no experience playing brass instruments. Someday. Anyway I think it’s the same idea but cause since can’t really change the length of the tube it’s limited to just one fundamental.

It is. Unlike other instruments brass instruments have a limited number of keys, some don’t have any. Take the trumpet - 3 valves, or the trombone - a simple slide. To get different notes a player changes their embouchhure (shape of their mouth) and in doing so cycle through the harmonic series. The 5th is the second…