Is it different if the design files are in one or several themes?

perhaps a stupid simple question, which is not straightforward for me:

which one is better?

  • adding the design of the site in several theme sections in admin panel and then add them to the default theme together.

  • all the css & javascripts (for the design) & header/footer in the same theme (perhaps a long code) in the admin panel.

are they different? and do they affect the site function?

You split stuff up if:

  • You want to make it easier for future editor to find the right component to edit. (one giant bundle makes stuff a bit harder to find)

  • You want to reuse theme components between multiple top level themes (like dark and light using an analytics component or common footer or something)