Theory seems to be the hot topic right now.  Rick Beato has some really deep music topics and great videos.  This is one that kind of gives an overview about bass, cello, viola, violin and how they can be used to form chords, like really BIG chords..no laughing.. Whole sections kinda stuff but breaks each instrument down into its ranges and where they fall on a keyboard.  Then uses the keyboard to demonstrate ...at the beginning he lays some ground work then gets into double stops and such and how each section can be used.   Geared more toward composers but still some interesting stuff.