Where does the concourse ci create containers to run the task? How to get into any of those containers?
fly intercept (aka fly hijack) seems to be what you're looking for.
https://concourse-ci.org/builds.html#fly-intercept
An important note that the container needs to still be running on the worker. For example, if the build has succeeded or failed awhile ago, Concourse might have cleaned up that container.
If this is the case, start your build again and once it's running you'll see it as an option to hijack when you use the CLI.