having a build rate of 30, compared to a T1 engineer (build rate of 5) means that whatever the T3 engineer is making, it will use 6x as much mass + energy as a T1 engineer making the same thing.
(assuming that there's no mass stall and no power stall)
And the same thing applies to factories. A t1 fac has a build rate of 20, a t2 fac has a build rate of 40. So if they're making the same kind of unit, the T2 fac will try to use exactly twice as much mass+energy per second.
But the exact amount of mass and energy differs depending on what is being built. So in practice, t2 facs tend to use more like 2.25x as much mass/second as a T1 factory if you're making t2 tanks instead of t1 tanks.
Units and buildings have build costs (mass cost, energy cost, and build time cost). A t1 engineer has a build cost of 260. You can take 260/20 to get 13, meaning (if there is no stall) it takes a t1 land factory 13 seconds to build a t1 engineer.
For each of those 13 seconds, the factory is trying to spend 1/13th of the total mass cost of the engineer and 1/13th of the total e cost
If you have two t1 engineers assisting the factory (each adding 5 build power to the factory's 20), then it would be about 9 seconds to complete the unit (260/30 ~= 9).
Buildings are priced the same way, in that they have a mass cost, an e cost, and a "build time" cost. Like a t1 torpedo launcher has a build cost of 450, that means a t3 engineer would need 15 seconds to make it (450/30 = 15).
One reason for a difference in mass/second and e/second is because different units have different ratios of total mass cost vs total e cost. Tanks tend to have a much higher ratio (more mass less e) and planes tend to have a much lower ratio (less mass more e).
The amount of "build time" needed to make a building is adjusted as part of how the game is balanced. If you need more engineers to get something built, that's a debuff. So it's a way to make something more expensive without actually increasing the mass cost. Or a building can be "buffed" by reducing the build time. For example, a relatively recent balance change was to reduce the build time cost for making TMD so you can build them faster in an emergency if tactical missiles are coming at your base. If you're not in an emergency situation you might not notice the difference, the important thing will be the mass cost of the TMD. But if there is an emergency, you're mostly worried about the build time cost.
Build time also is an issue for buildings that make things over time, like a strategic missile launcher or a strategic missile defense. And this is also adjusted for balance. A SML has 1500 build rate which means it would take 300 t1 engineers assisting it to double how fast a missile comes out (or: 50 t3 engineers).