Borderlands 2 – Drop Chance on the Badaboom

In playthrough 1, outside of the DLC, there's no such thing as scaling. Enemy levels are determined solely by the area they're in and your mission (certain missions effectively "level up" areas, notably Sanctuary and The Dust). You can easily check the level of an area by checking out the vending machines in the area, they sell items scaled to the level.

DLC in playthrough 1 scales to your level...ish. It maxes out at level 30. I played the DLC as a level 34 character and was disappointed to see everything was at level 30 in playthrough 1. Not sure if it has a minimum level it scales to.

Playthrough 2 works the same way until you beat the Final Boss; then everything in that playthrough scales to your level (except mission rewards for missions picked up before the final boss).

Playthrough 3 works the same was as Playthrough 2 after the final boss: Everything scales to your level, period. In team play, it will always be scaled to the highest level player's level. And you have to be at least level 50 to unlock it, so enemies are effectively level 50-61 at all times. Though if you take a mission before leveling up, the mission will still stay at that lower level, as always.

Basically the game works like a standard RPG without level scaling until you A) beat the game twice (because you should be maxed out anyway) or B) enter a DLC (because you can do that at any time so a preset level would be a really bad idea)

Yes; boss levels are treated like normal enemies levels in THVM, so as long as a boss is no longer part of a mission (meaning you've completed and handed in the mission) and you're in playthrough 2.5, the boss will scale with your level. Their loot, as always, is tied to their level, not yours, and capped at the level cap.

But effectively that means at level 50 in playthrough 2.5, repsawning bosses will be level 50(ish) and drop level 50(ish) guns. At all other times (during missions, before beating Warrior a second time) boss levels are locked in due to their mission/area levels, just like playthrough 1.