According to the artist, this takes place between part 7 and 8 of the series, see the links below.

Not sure why he would need income when he could literally just mug people with the app. But I guess that would be a crime.

By the way, if you like this series, I can highly recommend the JAV SDDE-379, as well as its sequels SDDE-419 and SDDE-439. Very similar themes and vibes to this series and the best JAV I know with that premise. If you know any others, I wouldn't mind learning of them.

Related parts that feature Yusa:
7: https://e-hentai.org/g/2270051/0095ccafc0/
8: https://e-hentai.org/g/2270244/3a9e8c0542/

Raw: https://e-hentai.org/g/2270223/345ff08a2d/
Source: https://serei.fanbox.cc/posts/2664833

Artist Links:
https://serei.fanbox.cc
https://www.pixiv.net/en/users/2622915
https://twitter.com/t_serei