For example: haste doubles your speed, boots of speed double your speed, and the Dash action doubles your speed.
Intuitively, it should be 2*2*2 = 8 times movement speed. However, the Dash action says "after modifiers" which implies something more like (1 [base] + 1 [haste] +1 [boots of speed]) * 2 [Dash] = 6 times movement speed. Unless it means modifiers to your base movement speed, which would be (1 [base] + 1 [haste] + 1 [boots of speed] + 1 [Dash]) = 4 times movement speed.
I know somewhere in the rules it says that for multiple applications of the same spell, only the strongest one applies, and that different spells with the same effect add together.
One of the sources here is an action, and another is a magic item. (Do magic items get treated like spells?)
The above is an example. The base question is
Do I add or multiply identical effects that multiply values?
I don't care what that value is (ex. speed, damage, money), or what it is from (spell, item, class feature, action, race ability); I'm looking for general rules for how to resolve stacking similar effects that don't give flat bonuses.