The mechanisms of parthonenogensis aren't fully characterized, but at a high level, it occurs in a secondary oocyte during meiosis II -- the oocyte only starts out with a Z or W chromosome.

It's perhaps best shown in this figure: https://rep.bioscientifica.com/view/journals/rep/155/6/image...

(Via this paper: https://pubmed.ncbi.nlm.nih.gov/29559496/)