Based on https://www.cnn.com/2018/10/15/tech/softbank-stock-saudi-ara... https://techcrunch.com/2016/10/14/softbank-and-saudi-arabias... it seems about half of that is Saudi money, a quarter of it is SoftBank money and while there are a few trust accounts among the SoftBank principal shareholders https://group.softbank/en/corp/irinfo/stock/ownership/#sec-0... they do not seem to have a large percentage. How bad it would be if the Saudis would lose a few (ten) billion dollars, that I can't tell.

Arm Holdings is held fully by SoftBank and the Vision Fund, should they fail, i am not sure what would happen but my guess is Arm would get out of the tumble relatively unscathed, it's too valuable.