Every good musician brings a pencil to rehearsal.

In Stanford Symphonic Chorus, Steve Sano began every new piece giving us a set of pencil marks to make in our scores. Nearly always, for a phrase that ends with a quarter note, we'd be told to replace that with an eighth note and an eighth rest. This is his choice, not the composer's.

Yes, crescendo looks like a < stretched horizontally based on the length of time that the rise should happen, decrescendo is >.

There are musical symbols for pretty much every common (and uncommon) instruction, and any gaps are typically filled in with textual notes.

However, conductors are tasked at interpreting the piece, which sometimes they might want a different sound, or perhaps the orchestra has trouble adhering to a tricky bit of notation. Any number of reasons will result in extra added notes jotted down in the scores directly.

Given that sheet music is often pre-printed or otherwise very difficult to edit, this is the next best thing.

Not every interpretation of a piece is the same.