It's most likely Scarif. Earlier in the series, in the episode "Aldhani", Lonni mentions in an ISB meeting that there's been increased traffic near Scarif. According to the starwars.com trivia gallery for the episode:
The Abrion sector and Scarif are mentioned in Supervisor Lonni Jung's ISB update as a site of increased traffic. There's a good reason for that, as revealed in Rogue One: A Star Wars Story: Scarif is the current construction site of the Death Star.
(Source: https://www.starwars.com/andor-s1-e4-trivia-gallery?image_id=5e9d99f9ab34eebd6b4f2a72; emphasis added)
This entry in the supplementary materials for the show at least shows that it was the showrunners' intent that, as of partway through the first season, the Death Star was being constructed over Scarif, so it would make sense for it to be in the same place by the end of the season.
(Note that contrary to starwars.com's claim, Rogue One doesn't actually depict the Death Star as having been constructed over Scarif, but concept art for the film shows that it was planned to be shown at one point.)
The starwars.com Databank also describes Scarif as "the principal construction facility for the vast Imperial war machine", though presumably it means this in the general rather than singular sense.