Do this in your site config:
[markup.asciidocExt]
extensions = ['asciidoctor-bibtex']
workingFolderCurrent = true
See https://gohugo.io/configuration/markup/#workingfoldercurrent.
That will allow you to use this structure, where the .bib file is adjacent to the .adoc file.
content/
├── examples/
│ ├── example-1/
│ │ ├── index.adoc
│ │ └── references-1.bib
│ ├── example-2.adoc
│ └── references-2.bib
└── _index.md
You might also want to do this:
[[module.mounts]]
source = 'content'
target = 'content'
excludeFiles = '**.bib' # do not publish .bib files when building the site
See https://discourse.gohugo.io/t/using-bibtex-files-with-asciidoc-content/56119.