When rolling HP for monsters, is 1 the minimum? Take the Gas Spore for example:

Hit Points: 1 (1d10 - 4)

What happens when a 4, 3, 2, or 1 is rolled on the d10, especially when said creature explodes violently when it reaches 0 HP?

The only reference I've found for this is that the minimum damage for any attack is 1. For example with a Bat who has -4 to their Strength:

Bite. Melee Weapon Attack: +0 to hit, reach 5 ft., one creature. Hit: 1 piercing damage.