There exist multiple spells/effects/mechanics that can trigger on the start of a character's turn. How in general should a DM rule the ordering, in cases where the order will matter?
As an example: Alice has been downed. She has 0 hit points and has been making death saving throws for a few turns now. She currently has 2 successes and 2 failures. She is also in the area of effect of a Cloudkill spell, which crucially has the following effect:
When a creature enters the spell's area for the first time on a turn or starts its turn there, that creature must make a Constitution saving throw. The creature takes 5d8 poison damage on a failed save, or half as much damage on a successful one.
At the start of Alice's next turn, one of two things could happen, either:
The death saving throw goes first, in which case she has a straight 50/50 of surviving this situation (for now!) or dying right then and there. If she is lucky and gets her third success, the cloudkill is still a concern, but not a matter of life and death. Worst case she is put back into an 'unstable' state but her death saves are reset, buying her precious more time to be saved or evacuated by her party.
The cloudkill check goes first, she dies instantly. Since no matter how well she does on the save to resist it, at best she still takes half damage, and any amount of damage is enough to permanently kill her whilst sitting on two failed death saves.
So whilst in many cases the order of 'start of turn' effects might not matter, in at least this example we have a case where the permanent death of a character could be drastically effected by the order ruled by the DM.
So how should a DM rule the ordering of such effects?