Hi, how come my video doesn't fit the screen entirely? And what can I do to fix that?

What are the dimensions of your clips and of your project?

If they don’t match you typically end up with black bars on the sides or at the top and bottom. You can, in the Inspector, set Spatial Conform to “Fill” instead of the default “Fit”.

Or it could be that you are seeing black bars when playing full screen on your mac. That is pretty normal because HD or 4K are typically 16:9 and most mac displays are 16:10.