Think about designing a building. How strong do the walls and beams and ceilings need to be? You figure that out with math, knowing the weights and strengths of the materials. How much of the materials do you need? It depends on the sizes of the walls, rooms, ceilings, all of that is math. Is concrete cheaper, or steel or cinder block? It depends on how much of each you need, what they cost to buy and build. How long will each stage of the construction take, and how can they be planned to overlap to get the building done the fastest? How much does each sq. foot. of the building rent for, and how long will it take to pay off the investment? How will it affect traffic in the neighborhood? How big does the garage need to be to hold the cars of what percentage of the people who work in the building or customers/clients who visit the building?
What about windows? How many sq. feet per wall? 567. How much heat will build up in the rooms because of the windows? That will affect how much air conditioning you need. Also windows leak heat during the Winter so more of them means more heat too. How many electrical outlets do you need? How many amps do you need from the power lines? How many circuit-breakers? How many lighting fixtures do you need? In modern buildings, the lighting system is considered part of the heating system--which is why you see lights on in skyscrapers in the middle of the night when nobody is there.
For big, tall buildings, architects are even concerned with how the wind affects them, how it swirls between buildings. They use computer models for this. Also things like earthquake resistance.
There's almost every kind of math here! Geometry and trig, calculus, accounting, engineering, finite element analysis, computer modeling, etc.etc.