Unfortunately extended stems in score

can someone explain why i am getting the stems connected across multiple instruments or at least how to stop it.

It looks like the combination of staff size and page size aren’t quite right for the number of staves in the system. That also looks like chords belonging to the same instrument, it’s just hard to tell because of the overlapping staves.

To change the page size, see here. For the staff size, see here.

I agree with Lillie.
It looks like the selected notes are all in the viola. To move the lower notes to the cello…
Edit>filter>notes in chord>bottom note.
Then press alt-M

(This situation possibly occurred because you did not extend the caret to include the cello during input?)