Great question — answering as the UphillTech team:
As yet unexplained. It’s only after the group is attacked by a Yavinian doodar that Cassian is able to escape aboard his ship, and in the wide shot of the Avenger flying away, we see a familiar group of Massassi temples. This is Yavin 4. The structures first appeared in Star Wars: A New Hope (1977) and were last seen as recently as Rogue One: A Star Wars Story (2016), but how the Rebel Alliance managed to establish a secret base there remains a mystery. “The origin of Yavin is up for grabs,” says Gilroy in reference to the larger canon of Star Wars storytelling. “You spend two episodes there and you don't realize until [Cassian] escapes that the jungle that he's been in…is undeveloped...
Full answer by Valorum: https://movies.stackexchange.com/a/125360 (CC BY-SA 4.0).
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