When there are treble ("G") clefs on both staves, it means that both hands are playing notes in the treble (above middle C), where most of the notes are in the treble and none (or hardly any) in the bass below middle C.
The piano uses both clefs.
Usually, the left hand is bass clef, the right hand is treble clef. However, parts for the left hand can cross over into the treble clef and vice versa.
Incidentally,...
All triangles are monotone (they only have one pitch). It depends on the size of the triangle (whether it is larger and lower pitch, or smaller and higher pitch)
You can play any instrument in any clef, you just need to be able to understand that clef. But yes, saxophone sheet music is generally notated in the treble clef. Well, at least I know for sure that...