Re: underwear -- yes, really. Some folks here do wear them under bike shorts with no problems, but most do better going commando. Bike shorts with a chamois are designed for you to wear nothing else with them. Underwear adds seams and can bunch up, which can both cause friction problems, and it also gets in the way of wicking and antibacterial properties of the chamois.
As for chamois buttr and other products, I think that varies. Basically it helps anywhere that you have problems with fabric or seams irritating your skin from constant friction. I generally only have problems just south of the sit bones, where the top of my legs/edge of my butt hits the edge of the saddle, so I put chamois butter on my skin there before putting the shorts on. I generally don't put it on the chamois itself, but I think some folks do. Back when I wore shorts that had a 2-piece chamois with nasty seams under my butt, I coated the seams in chamois buttr to make them less irritating.
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles