Functions like helm-org-in-buffer-headings,
helm-org-agenda-files-headings, etc. are built-in to the helm-org
package (they used to be part of the helm package, but now they're in
this separate helm-org package which is not installed automatically with
the helm package).
The packages org-rifle and org-ql are also helpful here. For example,
with the command helm-org-ql, you could use a search query like:
property:CUSTOM_ID=emacs-notes
You could also define a custom predicate that searches CUSTOM_ID
properties, so you could type a query like:
cid=emacs-notes
You could even go a step further and define a custom command that used
that predicate by default, so you could simply type the custom ID as the
query. See discussion at <https://github.com/alphapapa/org-ql/issues/61>.
