Lets use an example through this study: the Martial Warrior (Marty) has been knocked off of a cliff, and falls 100 feet the the bottom, into the square of a Cleric (Clarence). We will assume the DM rolls average damage. Let's break down this fall in order from general to specific:
A fall from a great height is one of the most common hazards facing an adventurer.
At the end of a fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell, to a maximum of 20d6. The creature lands prone, unless it avoids taking damage from the fall.
This is the original rule for falling, and the most general rule to follow. In this case, Marty takes the full damage of 10d6, 35 bludgeoning damage.
The second creature here gets to make a saving throw to avoid halving the damage. Clarence fails their save, and splits the damage with Marty: They both take 17 bludgeoning damage.
Beginning at 4th level, you can use your reaction when you fall to reduce any falling damage you take by an amount equal to five times your monk level.
The damage is reduced "when you fall," not when you take damage. This means the damage dealt by the fall is decreased, instead of reducing the damage once it is taken, through resistance, mitigation, division, diversion, etc.
In this specific case, Marty is now a 4th level Monk. If he falls onto Clarence's space and Clarence fails their saving throw, then the damage of the fall is split between them. However the specific wording of Falling on a Creature states:
...and any damage resulting from the fall is divided evenly between them.
So the damage is first reduced by the Marty's slow fall, and second split between the two. 35 damage is reduced by 5 times Marty's Monk level to 15 damage, which is split between them: both characters take 7 bludgeoning damage.
If a creature or an object has resistance to a damage type, damage of that type is halved against it.
It turns out that Clarence was with their friend the Abjuration Wizard (Abby), who happened to be showing off her new Stoneskin spell, giving Clarence resistance to non-magical bludgeoning damage. This means the damage taken by Clarence is halved, and only the damage taken by Clarence. So after Marty falls 100 feet, Clarence fails to get out of the way, and Marty's Slow Fall Feature takes effect, finally the damage that is taken is halved last. This means Marty would take 7 bludgeoning damage from the fall, and Clarence would take 3 bludgeoning damage. If Marty had the resistance instead, this would still not be translated into reduced damage for the other person, as the resistance's damage reduction is applied after the damage has been dealt.
Other Specific rules: Various Subclass abilities
All of the subclass features you have listed are great examples of damage reduction, and they all have a similar piece of wording that is different from the Monk's Slow Fall.
If you are raging and another creature you can see within 30 feet of you takes damage, you can use your reaction to reduce that damage by 2d6.
Bastion of Law:
When the warded creature takes damage, it can expend a number of those dice, roll them, and reduce the damage taken by the total rolled on those dice.
When you or a creature you can see takes damage while within 10 feet of the tentacle, you can use your reaction to choose one of those creatures and reduce the damage to that creature by 1d8.
Song of Defense:
When you take damage, you can use your reaction to expend one spell slot and reduce that damage to you...
Unlike the Slow Fall ability, which takes effect when you fall, all of these features take effect when the creature takes damage. These would all reduce the damage after impact, when both the falling creature and fallen-upon creature have already taken the damage.
Thankfully for Clarence, his Raging Ancestral Guardian Barbarian Friend (Barb) is nearby, and she uses her Spirit Shield ability to reduce the damage of the fall by 2d6. Unfortunately for Clarence, Barb rolls garbage, and only reduces the damage by 2. According to the order of operations in the PHB, the damage would be reduced first (from 7 to 5), then halved by resistance, resulting in a final devastating total of 7 bludgeoning damage for Marty, and a brutal 2 bludgeoning damage for Clarence.