It may be grainy because the Nokia 7 Plus noise reduction is not overused. Noise reduction can be bad for night photography.because it softens too much.
In low light you need a low ISO. Go to pro mode to set you ISO between 100-400. Low ISO = less noise
Keep it mind smartphone cameras in low light are never good.
Try to upload an sample. Use dropbox or whatever and put the link. Be sure to post the full resolution picture. you can post directly on the forum but I'm not sure is a big picture will be accepted.
Or, you can compare yourself with photo from GSMArena for example. They uploaded full resolution samples on their Nokia 7 Plus review.
Too much noise = ISO too high
Blurry = shutter speed too long / adjust focus / make sure the lens is clean.