[RPG] What happens to dying PCs when combat ends

If you think about dying in terms of rounds and what that means for time, a death saving roll happens every 6 seconds. This means that a character knocked out has a minimum of 12 seconds (one roll of 1 for the double fail, and then a roll below 10) and a maximum of 30 seconds (2 successful rolls and 3 failed rolls) before they die (assuming no additional attacks and such).

Because this happens pretty quickly, I would keep turn order going after the combat is resolved. Your party might only have 6 or so seconds to get to each player and stabilize them. This can pose a challenge for the long ranged magic user to make it to the front to stabilize the fallen fighter, or vice versa. It's always possible they fail the stabilization check too, making for those 6-30 seconds to be pretty intense.

I find a bonus is that this additional last chance to die prevents some players from using kamikaze tactics where they think "who cares if I get knocked out? I'll still be stabilized right away." Not necessarily in my games.

The PHB (pg 198) has that section written as such:

Most DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws.

Mighty villains and special nonplayer characters are common exceptions; the DM might have them fall unconscious and follow the same rules as player characters.

Note the use of most and common exceptions.

Additionally, PHB 197 has the following:

The best way to save a creature with 0 hit points is to heal it. If healing is unavailable, the creature can at least be stabilized so that it isn’t killed by a failed death saving throw.

You can use your action to administer first aid to an unconscious creature and attempt to stabilize it, which requires a successful DC 10 Wisdom (Medicine) check.

Note the use of creature here, not you or player character.

That makes it clear that having an enemy die instantly at 0 is an option that the DM may use. It's completely legal to rule that monsters use the same rules "Dropping to 0 Hit Points" as PCs. If you want to make sure that some of your enemies are alive after the battle to interrogate them, you should:

• Make use of the "Knockout" rule, when able.
• Otherwise, ask your GM to determine which enemies are still alive; (s)he can determine that by fiat, or by rolling death saving throws for each of the enemies.
• At this point, you should know which enemies are alive but dieing, and which are at 0, but stable, and you can move on from there.