What kind of insulation to use for sound dampening?

[Bruce P.]

Hi, my friend is building a ~650 sq ft detached ADU in Los Angeles. Since it's new construction, what type of insulation (and/or other techniques) would you recommend to "dampen" sound coming into the unit (as well as out from the unit) so they don't hear the next door house and vice versa? (It would be also nice to keep the ADU cool as well in the summertime, my understanding is having good insulation can do this, please tell me if this is wrong....)