You can purchase worlds individually here. However, The All Access Pass is better value because you gain access to the world you want and everything else on this website.
Every world has a Bedrock Edition version and a Java Edition version which allows for an easy installation process on PC, Mac, IOS and Android and Xbox. On Playstation or Switch, the worlds can be hosted as a realm then joined from your device. We have installation instructions for each one of these devices. Find them here
Yes your membership can be cancelled at any time within your dashboard. Once cancelled, you will only have access to current and future worlds for the monthly period you paid for.