A lot of your questions seem to require a little more context:
1. It really depends on what you mean by "scale" I/O, data, access? I use MongoDB Atlas, it's online, easy to manage. MongoDB, SQL, Snowflake all work, it just depends on your application and skill level.
2. I would recommend Stripe, but honestly, PayPal can process transactions just as well.
3. I took an angular course on Udemy,...