Ambri's been building towards this for a while, so there's a lot of material elsewhere on how the contents stay warm - put simply, it's well-insulated, and as the battery gets bigger, the square-cube law makes it more and more efficient in that regard as the volume of heat stored increases much faster than the surface area the heat can escape through. A cold start would be expensive in terms of energy, but Ambri's website states that a daily charge-discharge cycle provides enough waste heat to keep the critical components molten.

Just Have A Think did a piece on these batteries a couple of years ago, if you're interested to learn more.