How can I format parts of a bibliography entry, e.g. making the title bold=

[Y.D.X]

It depends on the font-weight attribute in CSL.

(If you need further help on editing CSL, please tell your bibliography.style.)

@syqwq

is there a way to access the ingredients of a bib entry?

You can use citegeist – Typst Universe or A snippet to debug bibliography entries by showing what CSL receives. However, they won’t affect how typst layouts the bibliography (unless you use magic show "…" rules).