How are standing waves and resonance applied to musical instruments?
Standing waves are themselves an example of resonance. Musical instruments are designed to produce standing waves, so they are designed to produce resonance in that way,
Standing waves only occur at specific frequencies which are related to the natural frequency of the oscillating system, e.g. guitar string, clarinet air column. So the resonance phenomenon is what allows for notes to be produced on a musical instrument at all.
A guitar though makes an additional use of resonance. The body of the guitar is carefully designed to resonate when notes are played on the strings. The reason is that the body can then behave as an amplifier for the strings. This amplification aspect of resonance is true of other musical instruments too.