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