Insert Minecraft HD skin?

I have a Minecraft skin with over a thousand times something pixel and would like to insert it into Minecraft. Someone has already asked this question only I do not understand what he means by the answer. Here is the question link:

Thanks in advance

That is not possible… The MC skinks are always limited to a few Xpairs of pixels, you can't simply use an HD image for this, you can forget

But how do the YouTube do it and how does the guy from the link do it? 🤔

By using client modifications like Badlion or texture packs.