Those are there because the content creator you’re watching decided to put them there. It’s entirely up to them whether they show up and when they show up.

And they can be hidden, so it's not exactly entirely up to them, nor should it be. If they wanted them in the video content they could put them there.

how can they be hidden?

Most easily using an add-on like Enhancer for YouTube[1], which has been hidden in addons.mozilla.org[2] but is still available on the author's site. But the player is HTML so writing element hiding rules (like `youtube.com##.ytp-ce-element`) is the easiest, which I figure out the relevant element using the Developer Tools Inspector tool, but the uBlock Origin Element Picker tries to make that easier. In this case the .ytp-ce-element-shadow is chosen first in the uBlock picker and then you can pick the element class I mentioned first, but it's nicer to have fewer rules to effect the desired outcome.

[1] https://www.mrfdev.com/enhancer-for-youtube

[2] https://www.mrfdev.com/contact