Aspect ratio in FCP

This has nothing to do with youtube per se.

There are two separate things to be aware of.

1) does the video itself have black bars in it or not: this depends on whether the clips it contains match the aspect ratio of the project.

If you are unsure, the simplest way is to change the background in FCP from black to checkerboard.

You can change the aspect ratio to match most of your media, and you can adjust Spatial Conform of some media (for example, from Fit to Fill).

But most likely your “problem” is

2) the exported video shows with black bars on top and bottom when playing in *full screen* (in youtube, but also in Quicktime Player or other software).

This is due to the aspect ratio of your project not matching the aspect ratio of your display. Different people have different displays so you can never guarantee that it will match. Case in point: the usual HD or 4K resolutions have an aspect ratio of 16:9. Playing that on an HD monitor completely fills the screen. But playing on a typical MacBook screen still shows bars at top and bottom: not because they are part of the video button because the screen is taller, with an aspect ratio of 16:10.

You should use the aspect ratio that better matches your creation. Most video recorded today is in 16:9. If you use, for example, video recorded on your iPhone is typically 16:9. Most video uploaded to youtube (if not the **** vertical ones) is 16:9. Something has got to give. If your sole concern is to completely fill your 16:10 MBP screen , then sure, make it 16:10. But then any 16:9 video that goes into your production will be scaled and cropped… If your audience is greater than one, use 16:9.

If you mean on youtube, in fullscreen, in your MacBook Pro screen, yes, that will probably work.

But if you want the video for yourself, why not just export file and watch in Quicktime Player?

If you someone else watching on youtube, then DON'T. You have no idea what screen people have - most people will probably watch on an iPhone, or an Android phone. Others may watch on an iMac, or a TV, and these will have different aspect ratios.