Hi @Tobias_Bar and welcome to the forum!
Yeah, this is kind of confusing, you’re understandably stumped.
What look like sections in subtasks aren’t actually
sections in the technical sense.
Sections are a wholly different type of object from tasks, and
sections appear only at the top level of projects.
Those things that look like sections in subtasks are actually just a special type of
task object called a
separator, and you can create one by setting this boolean property on a subtask:
is_rendered_as_separator = true
(If you’re curious about why sections and separators look like they are the same but are totally different from a technical perspective, let me know and I’ll explain; OTOH if you don’t care and just want the darn section/separator in your subtasks, then follow the above guidance and you’re all set! )