On Sun, Apr 3, 2022, 4:29 PM <c.buhtz@posteo.jp> wrote:
Hello togehter,

is it usual to have auto-fill-mode (automatic new-line after n
characters) turned on in org or org-roam (including fill-column set e.g.
to 80)?

I cannot comment on that. Enabling auto-fill-mode is user preference. 

Or let me ask from another point of view: Is it usual to have multiline
list entries?

Yes, if auto-fill-mode is enabled in org-mode. 

I know that org is not markdown but I tried some markdown editors to
test it. They still recognize the linebreak in my first example and
rendered it "correct" as a 3 item list.
I checked the syntax docu [1] about it but I couldn't find a hint or didn't understand.

Do I understand it correct that the end of a list is marked by an empty
line?

Yes. I think that the list items follow the same rules as Paragraphs [1] i.e. A list item continues until it sees a blank line. This is identical to Markdown. 

Below, you have 2 list items:

- item 1
- item 2

Below is also 2 list items:

- item 1
  item 1 continued
- item 2

Below is also 2 list items, but the first item has 2 paragraphs:

- item 1

  second paragraph in item 1
- item 2

I hope that helps. 

[1]: https://orgmode.org/manual/Paragraphs.html#Paragraphs