How is it decided where to place Roads?

It depends a lot on how old the road is. Several answers manifest in Manhattan.
In two parts of town (Financial District and Grennwich Village) the roads are over 200 years old, way before cars, and they tend to follow either the lay of the land or shortest distance from A to B or the wall built to keep the pigs out (Wall Street… how ironic, pigs on the inside now). Broadway follows the ridge line of the island. Canal street is along a low zone that was swampy before they built a canal for cross-island transport (subsequently filled in).
Then there’s the modern stuff, the grid plan. The numbered streets and avenues on a systematic grid that ignores topology.

On bigger scales you’ll see roads heading in/out of urban centers and ports and to/from the pass in the mountains.
As far as inclines a road will tend to be laid out to even out the climb to the pass.

City planners help do the big roads, and plan where residential vs commercial property will be. Inside of communities the developer decides, gets it approved by the local government, and then many times gidts the roads back to the city after the subdivision is built. Sometimes they stay private roads.

In America the interstate system has a lot of logic to it. Odd numbers run north south (I95) and evens run east west (I10). The north south are lowest numbers in the west getting bigger and bigger going east. So, I75 is west of I95. I10 is south of I40.

Sometimes land is take through eminent domain, which means the government takes the land from property owners. They pay the owners, but many times this is met with a lot of resistance from owners and local communities. People can fight the government andnplans for roads, and sometimes they win.

In the suburbs, much of the sub division layout is originally designed by the developer, who takes the plan to the local planning agency. But then everyone gets a say, including all the utilities, such as sewer and water districts.

The streets in Boston are much crazier than New York, and much of it was paths for cows that went to the Back Bay area to graze. That street layout is 350 years old.

San Francisco had a few fires in its early post gold rush existence which burned the town, and allowed the city to start over on an orderly grid.

Big highways and interstates try to follow terrain and existing corridors to minimize taking over land and also keeping earth moving to a minimum.

In the UK, a lot of the Roman roads are either still in use or can still be made out in the land. Otherwise the roads followed the edges of fields or the lie of the land, eg along a ridge or a valley. There are just a few places in Britain that have a grid system but apparently it was invented in Edinburgh when they had a competition to design the New Town in the 1700s.