MongoDB Atlas is still good enough, flexible and powerful API, with 500 MB storage. However, if you need regular backups, or pro support, it is at least almost 20 USD / mo.
Firebase Storage (not only Firebase Auth). I never have to pay for it. I think AWS S3 in the same category.
Similar to MongoDB Atlas, mLab provides a decent MongoDB cloud hosting free plan if you're looking for an alternative.
yup, and that 😉
I like Atlas, 512MB is more than enough to get started 😉