I figured out that terraforming is crucial in setting up your buildings. If you dont take care of that before plopping down the buildings, theres always going to be a gap between them to form the slope.

So usually the first step in any new game I did, (I failed a lot because I wanted to start on the hard option for starting money) is to build a road vehicles depot and buy an excavator (having it in a construction office works too, though when its used for any construction in it, the excavator is not usable for terraforming; the big excavator for dollars is the best one, for 9k $ though). With the excavator present any future terraforming is practically free (except for fuel for the excavator, it needs a working fuel station to refuel (this isnt needed, when "vehicle energy management is" not chosen for the game) Just keep in mind to uncheck any auto-build option for this to work. Also the more free excavators you have the quicker you can terraform. The max amount of excavators to use at once is 7 though

The provided tool for smoothing down the ground is okay, but i found another way to get a smooth area much quicker and more reliable. To do that, find the place where you want to start building up and terraform it with a building. Don't plop the actual building down if you got the building lot smooth. The next step is moving the building towards the edge of your smooth area while keeping the center of the building (red square) on your smooth area and terraforming a new building lot, adding smooth area to the already existing space. I usually use the steel mill for that, because it has the largest footprint. The technical university building is great when you need a straight line for streets or rails.
E voilá, you have a huge, open and smooth area to build your next city/industry. Just keep in mind that you need to do that before building any infrastructure there (including roads, paths and power lines) because any building/infrastructure has a "no terraform" zone around it.
You cannot move entire hills in one go doing that. For getting rid of cliffs you need to lower/raise the land before smoothing it with a building.

Recently I also found out, that if you keep excavators in a disconnected construction office, they will still work while being out of fuel.