How to add curvy segments to objects? Assume the following model is the beginning of a car-like thing.



Now we want to begin adding the windows and roof. I want them to form a nice smooth curve on the edges, and came up with the following solution:

I create a bezier curve, align it as good as i can with the help of orthographic camera. then i extruded it to get the following:



After that I convert the curve to a mesh, join it with the car mesh and run "merge vertices by distance" to join the "alignment points". This looks so messy and it actually is. Now i have much n-gons, and it only gets a bigger mess when i add the missing faces. 

How can I do such things in a better way? What is best practice here?
The best practice is to take the car you need to model 1 part at the time. If you notice the chasis is broken down into real life individual pieces. The door, the lights, everything is made out of pieces. Also, use references. Then just place objects closer together. No one looks for seams (that are not natural part) of the car's modeling.