Every account gets one Free server. Paid plans get more CPU and RAM, run 24/7 and can use your own domain. Add as many servers as you like to your basket.
Your server can briefly use extra CPU for things like world generation. If it stays busy for more than a minute it drops back to its base speed until things calm down.
They run for up to 8 hours at a time and go to sleep sooner when nobody is online. Then they stay off until you press Start in the panel.
Any time, from your account page. You won't be charged again and the server keeps running until the end of what you've paid for. After that it stops, and its files are kept for 14 days.
Yes. Open its panel and press Upgrade / renew. If it already renews, you only pay the difference for the rest of the month (or get the unused part off your next bill when moving down), and your world and files stay exactly as they are.