Upgrades stack before they are multiplied with the base income
So if you have +15% from tech 1, +15% from tech 2, +20% from tech 3, +50% from your advancements and +25% from an artifact (just some random numbers), you would get a multiplier of: 1+(0.15+0.15+0.20+0.50+0.25) = 2.25
However, 20% bonus from watching ad is exception here, as it's 'real' 20% (that may be a bit misleading)
Without that last +20% tech, you would get a multiplier of 2.05, so the actual increase in multiplier is +9.8% and not the +20% you think you're getting.
It would be nice to see both numbers in game