How do I change the appearance of my slot bar and my inventory in Minecraft?

I recently saw in a YouTube video that the player had changed the look of his slot bar. I wanted to try that too, but couldn't find any instructions on the internet, so I'll try it here.

Texture packs but if you mean different then with mods there are enough tutorials for it

Texture packs I think

Search for texture packs or resource packs on youtube.

You do that with texture packs.

Either you take a finished one, or you do it yourself.

You can do this with texture packs. You can easily download and use it. If you have something special in mind, you can also put together your own, but that is a little more complicated.

I find PureBDCraft the most beautiful texture pack, you can have a look: https://bdcraft.net/

You can also find instructions there or lots of them on YouTube