How does a trombone work?
The trombone plays in concert pitch: when the trombone player plays a written C, the note that comes out is the same as a C on the piano.
Unusually, the trombone is written in many clefs, but most players begin their journey in the bass clef, playing notes that look and sound like the notes played by the left hand on the piano.
The only real exception to this is in British Brass Bands, where the tenor trombone plays in Bb treble clef; as long as the player plays brass band music, this isn’t a problem, but other ensembles tend to stick to concert pitch in C bass clef.