I recently started making youtube videos again and needed to reduce the background noise.
Luckily, iMovie makes removing the background noise simple.
Steps to reduce background noise with iMovie
Inside iMovie, Click on the audio or the video where you want to reduce the background noise.
Click on the bars icon on the top right side of iMovie (screenshot below)
Check "reduce background noise" and adjust the amount. I recommend somewhere around 50% and 70% for it to still sound natural but play around.
That's it! No need to use fancy audio software to reduce the background noise. iMovie does a really good job of it.