There is no razor tool in Media Composer! In order to cut/split a clip, you want to use ADD EDIT 👌 Fine tune your position with the arrow keys. For video editors coming from Adobe Premiere Pro, Final Cut, and other NLEs – many start looking for the razor tool to cut-up their clips and get confused when they cannot find it.
In Media Composer, there is no razor tool. Instead you use the ADD EDIT button or shortcut key on your keyboard. To cut a clip, park your playhead where you want, you can always fine tune your position with the left/right arrow keys, make sure you select the tracks you want to cut, and then click the ADD EDIT button in the toolbar of your timeline window. Or, even better, use the keyboard shortcut which is shown in the video.