It is important to understand the difference between files and pages fields on the one hand, and files and pages sections: https://getkirby.com/docs/cookbook/setup/kirby-in-a-nutshell#sections-or-fields
You cannot mix fields and sections.
sections key, you introduce that what comes are sections. So if you wanted a files section followed by some fields, you would have to use a files section and a fields section, like this:
This would make sense if the files section was intended to upload files.
In your second snippet, the
video thingy is not a section but a field, which allows you to select files which are then stored in the content file.
Apart from some special fields like the info, headline, and line fields, all fields store something in the content file.
Sections on the other hand, provide basically functionality, like uploading and displaying files, creating and displaying pages etc.