12" MacBook battery is dead after 386 cycles
An inflated/bulging battery is a safety risk and you should get it repaired ASAP. The reason for this could be anything from manufacturing defects to improper care.

If you got AppleCare when you bought your Mac, you may still be able to replace the battery for free. Otherwise, Apple won't change the battery for free, as warranties only last one year.(see here)

If you schedule a repair without AppleCare or warranty, it can cost anywhere from $129-199, depending on your Mac's model. You can schedule a repair here.

Or, if you are willing to take the risk, you can replace the battery on your own using an iFixit guide.