Hi. Thank you for the additional questions!
Juju is built with cloud images like the ones at https://cloud-images.ubuntu.com/ in mind. These images are designed to be used with OpenStack clouds, and are similar in nature and configuration to the instances used in AWS.
The cloud images generally have some additional packages installed, and are designed to work well with cloud-init, which is an industry standard tool that solves some of the problems inherent in bootstrapping a software image in the cloud, without human intervention.
Since a human operator isn’t going to be sitting at a cloud instance as it comes up, able to type in a root password, cloud images typically come with a user account built in. cloud-init will drop an ssh key into the image as it inits, giving the human operators who eventually want to interact with the machine a secure way of doing so, without the need for a password.
When you’re working with a manual provider, you may need to do some additional work to make your images look similar to the images used by the clouds that Juju natively supports. For example, you need to be able to run cloud-init, which is how the Juju machine agent gets installed, among other things. It can be a little bit tricky – feel free to ask questions here as you continue to evaluate Juju.