How are before_script/afters_script executed in relation to the script commands?

I understand what the before_script does. It executes commands to run before a job's commands get executed.
But how does that actually work under the hood?

Is it one "entity" that runs all the commands defined in the before_script script and after_script in the same process?
Or are these being executed by different processes (subshells?) sequentially?

If for instance in the before_script a command is run that in a normal setting, running in the background, could last the duration of the script would that be killed once the scope of before_script is finished?

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