How can I change the table of contents to include the relevant chapters in different parts?

[Andrew]

So now parts are headings. And chapters too.

@Andrew_Duncan

when you collapse part_1 in the table content, chapter_1, chapter_2, chapter_3 and chapter_4 shall appear.

What does “collapse part_1” means? Where you “collapse” it?

@Andrew_Duncan

So basically, chapter_1, chapter_2, chapter_3 and chapter_4 are children of part_1 but still look heading 1.

In what way exactly are chapters children of part 1? Where exactly the chapters still look like top-level headings?

In the PDF’s Document Outline, you can only collapse sections with actual children, that appear as children. In the Typst’s outline you can style them however, but to distinguish between them and make a proper Document Outline you must use different levels for different section/heading types.