How do the queues in my project work? Is it only possible to have one thing building/deploying at a time?

For example, if a developer pushes something on the stage branch, and it takes 10 minutes to build, can I deploy a hotfix on master in that time?

Environment builds occur in the order that they are received. If you push an update to two active environments, the build on the environment pushed first will have to complete before the second environment can be built.

Until that time, the second build will continue to show a state of either ‘In Progress’ or ‘Pending’ depending on the state of that first build.