How does deciding on the DC work differently within indie games?
Are players part of the process of deciding or is it GM decides/notes?
Different games handle it differently.
4e (basically "indie" D&D)
* "Subjective" Core Action Resolution means the DCs are codified, player-facing, scale with the level of the PCs, and are mathematically encoded to achieve a particular spread of results in action resolution (eg - a 67 - 75 % hit rate in combat). So if your PCs are level 5, your Easy/Medium/Hard DC will be x, y, z. Skill Challenges are codified to tell the GM what DCs are involved (eg a level 5, Complexity 1 Skill Challenge will require 4 * Medium DC obstacles to be overcome). Stunting/Hazards/Traps works the same. Monster math (HP, AC, Fort/Ref/Will, Damage) scales with PC level the same way (with adjustments based on type/role).
Powered By the Apocalypse
* Just like 4e, PBtA DCs are a subjective spread of numbers (eg, there is no DC based on of-world obstacle) built to create a mathematical distribution of results (a bell curve). However, due to the maths in PBtA games, they don't change at all (unlike 4e where the numbers aggressively move up, but the % chances are roughly the same, regardless of level). Roll 2d6 + modifier and compare:
6 or less = Mark xp and GM makes a Move against you (typically Hard Move)
7-9 = Success w/ Cost/Complication/Hard Choice (eg you get what you want but GM makes a Soft Move against you)
10+ = You get what you want
Forged in the Dark
* Like PBta but this is a dice pool game that includes the same spread but also a critical success and there is Position and Effect. Throw you pool and compare:
1-3 = Bad outcome. Things go poorly. You probably don’t achieve your goal and you suffer complications, too.
4-5 = Partial success. You do what you were trying to do, but there are consequences: trouble, harm, reduced effect, etc
6 = Full success. Things go well. If you roll more than one 6, it’s a critical success—you gain a boon/advantage.
Then you have
Position. Position is how dangerous or troublesome an action might be. If things go wrong, it tells you how wrong (and that scales). Position is Desperate (mark xp if you make an Action Roll w/ Controlled Position), Risky (normal), Controlled (danger/complications reduced). Position is determined by (a) the situational circumstance for the danger/trouble involved (look at it like Normal/Advantage/Disadvantage in 4e/5e), (b) a prior roll/complication (Position opens at Risky unless (i) you've got a complication due to a prior Action Roll and that complication is worse Position or (ii) some other aspect of system, eg Devil's Bargain, has been leveraged to increase the danger/threat
Effect is about Assessing the Factors involved in the situation of the Action Roll; Tier, Scale, Potency, Magnitude (for supernatural). If it tilts away from you or in your favor, you go from the default Standard to Limited/Great. Just like with Position, there are either ways this can be changed by player actions/resource expenditure or Costs/Boons in prior Action Rolls.
Mouse Guard
* Dice pool game like FitD. Assemble your pool and throw. Get a number off successes that equal or exceed the game's DC (Ob) and you succeed.
Two of the ways the GM sets Obstacle numbers similarly to how Effect is handled in FitD games; (1) Assessing Factors for a Skill in the Skill list and (2) by Assessing Factors in the Seasons and Territories chapters. Then there are versus/contests and that is determined by a roll of the dice from another player or the GM. DCs in this game scale similar to 5e but are called Ob (Obstacle). Roughly; Ob 1 = easy, Ob 2 = medium, Ob 3 = hard, Ob 4 = very hard, Ob 5 = nearly impossible.
There are plenty of other ways (
Torchbearer is kindred to Mouse Guard with subtle difference), but that is all I have for now.
The key thing that all these games share is (a) follow the rules, (b) encoded procedure/maths for setting difficulty and determining success/complication/failure and fallout, (c) everything is player/table-facing.
They all have varying other tech/procedures (eg Blades gives players a ton of capability to manipulate Position and Effect and adjust their dice pool via resources to martial and moves to make).