They are different organizations, completely different type of competitions. As far as I know, there is no international component to the science olympiad, aside from the participation of a Japanese team as a sort of ambassador team. The US-based science olympiad competitions basically run through the typical us school calendar, with the national tournament in late May or sometimes early June. As far as I know (our team has not won nationals during our tenure) the competition ends after nationals; the winning teams get the honor and admiration of their co-competitors, and often many event winners are offered scholarships or prizes from the host university and/or sponsoring companies or organizations (such as NOAA, the CDC, etc).
I’m not familiar with the junior science olympiad, but from my brief search, it appears to be very different, more akin to the subject-specific competitions.
In case this helps, here are a couple links to science olympiad (the US competition) sites.
https://www.soinc.org/ https://scioly.org/wiki/index.php/Main_Page https://www.scienceolympiad2020.com/
