I recently purchased Mordenkainen Presents: Monsters of the Multiverse. Inside, I found two specific races of interest. Githyanki and Githzerai. While they may not be obvious choices like Bugbear or Minotaur, they have one feature that instantly caught my eye. Psychic Resilience grants Gith a natural resistance to psychic damage. I'm pretty sure you know where this is going. Combine that with the Totem Warrior Barbarian, pick the bear, and while you're raging you will have resistance to every damage type, effectively doubling your hit points.

1. How is this even possible/allowed???

2. What do you think is the best race and Subclass for Barbarians?

Going with your theme, I would actually choose the Kalashtar race over the Gith races. Sure, their ability point spread aren't in line with barbarian, but the majority of their features tighten up those typically very-poor areas barbarians struggle with; advantage on wisdom saves, psychic resistance, etc. Not to mention their story and lore offe interesting plays on what it means to rage.