The doorknob and window looks very low effort. I know it’s supposed to be cartoony, but it could definitely be improved.
actually I want to give a realistic touch
If that’s the case, I suggest details on the door itself with depth, using either wedges or cylinders. It really helps your door stick out.
Well, if you do want the realistic style, you should probably use unions rather than meshes for the whole build. Looks like an inflatable.
well, meshes help me a lot with those constructions and sometimes those some fact unions use a union to make the circular part of the window
If it helps you, really you can use whatever works. Just make sure it can look as great as it can. I suggest reference pictures.
actually use a picture for reference and I gave it my touches
The key to realism is high detail.
If you look at the doorframe, it looks quite plain. You should definitely add some extra detail here, such as a pattern or some kind of depth to it - for example if you have an outline which sticks out slightly and sinks inwards towards the centre of the door. The image below might give you an idea:
If you look at the door handle, it just looks like a part and a ball which are both the same colour. If you want it to look realistic, you will most likely need more parts in order to flesh out the shape and the details of the handle. You can make use of cylinders and wedges instead of just parts and balls in order to more accurately define the shapes you create. The ball also looks unaligned in relation to the part, but I may be wrong about that and this may just be due to the image.
The outline of the door is also very basic; from the image it just looks like 3 parts. The hinges are a nice touch, and the window would be nice in a cartoon style build, but again, it looks too basic to fit your aim of realism.
On top of adding more detail, I’d strongly suggest you add more colours to the door to make it look more realistic. Think of doors in real life; lots of them aren’t all the same colour all over and they’re not completely flat like the doorframe in this image.
Those are all my points. It’s a decent start so far and I can see you’ve put effort into it. I’d like to see what you produce in the future should you decide to improve it.