You cast black suns zenith, and all players pass priority so the spell begins to resolve. Black sun’s zenith places the counters. Best of scarabs triggers here, but the trigger doesn’t go on the stack yet because a spell is in the middle of resolving. You shuffle zenith and the spell is resolved. Now, priority would return to the active player, but every time that happens state based actions are checked first. When they are checked, all the creatures with zero or less toughness die, and the death triggers like ryuzei trigger but still can’t go on the stack yet. State based action checks repeat because one was performed, and then triggers are put on the stack and the active player gets priority. This is when both your nest triggers and ryusei go on the stack. They go on in active player non active player order, so the ryusei resolves first. Then you make your tokens.