oops, the way you responded the first dude made me believe you were asking about AV, apologies. But yeah phys def is used mostly on stall as a better tool against the physocal attackers like Kartana and Tapu Bulu as it obviously has a lot more defense investment. It also gets Sleep Powder I guess, which is interesting but in general most of the time physically defensive Tangrowth isn’t quite worthy because you lose the matchup against the likes of Ash-Greninja and Tapu Koko while you don’t gain much on the defensive side. Which leads me to your next question:
So yeah AV Tangrowth can take on Zygarde mostly fine for as long as it can break a Substitute from Zygarde, be it with Hidden Power Ice or Grass Knot. Choice Band Outrage can 2HKO, but at that point you can just switch out and recover your HP with Regenerator. Dragoniun Z can be annoying for Tangrowth, but that set is pretty much unviable and even then you can EV your Tangrowth to eat.
You can read my post above on that :)
Hope I answered everything, lmk if I missed something