How do you remove background noise in a video so that the sound becomes smooth
Audio noise? With an external software like Audacity
Shotcut can do a part of it and it can be enought in some situation with the dedicated audio filters.
I recommend trying a new plug-in called Instant Dialogue Cleaner by Audionamix. It can remove a wide variety of background noise very easily, and is a must for cleaning up dialogue. I use it in my workflow for pro tools and it helps me take a lot of background sounds out. Here is a video on Youtube about it: https://www.youtube.com/watch?v=PUwrs_AsLJ0