ORAS had Hoopa rings that were, for the most part, in random locations, though a few did make sense, like Heatran and Lugia.

Gen 7 pulled Alternate Univer bullshit

Gen 8 simply threw them into a rougelike Raid Boss rush

Gen 9 had a guy giving out snacks.

I hope Gen 10 gives an actual lore reason for why returning legendaries are there outside of "it's just there".