Youtube fullscreen Huawei?

On my new phone (Huawei P20 lite) I can't use the whole screen of my mobile phone on Youtube (see picture).

Do I have to change something in the settings or something?

thank you in advance

You surely know how to zoom in on something or?

So fingers together on the display and then one finger to the right and the other to the left.

Oh thanks, but then something is cut off below and above

Yes, that's the disadvantage.

I own the Huawei P20 Pro

How old are you if I may ask 😂

I'm not old.

OK