for Youtube HD video I've seen it suggested to use 1920x1080 pixels...

When creating a new drawing in Krita it ask for Width, Height, and DPI....

The Question: So what is the best DPI for HD video on Youtube?

Thanks for any help.

dpi (dots per inch) and ppi (pixels per inch) are about the printing of images. In krita, which does not do printing, these figures are a guide to help you plan/determine the image size you need, depending on what your printing intentions are.
If you want to make a 1920 x 1080 video then just set the width to 1920 pixels and the height to 1080 pixels and that's all you need to do.

Ugh, I thought I’d find the answer here. Still, I hope there’s someone who can tell me which DPI I should pick. I want to create a channel to promote my business, so I need to know that damn DPI

Hi there. 72 dpi is recommended. I make a lot of videos for different youtube channels, and this dpi always works well. Actually, I’ve never tried setting another dpi cause I think it makes no sense, actually. As for promoting, what tools are you using for that? I have 5 channels with 10 million subscribers overall. When we launch a channel, we start with buying come likes on https://fanhype.de/youtube-likes-kaufen/. Then we start an SMM campaign where the SMM manager and the Graphic designer are involved. If you want to promote a product, don’t talk much about it.

DPI has no affect on digital images. DPI is for print only. A document that is 1000 x 1000 at 72 DPI will be the same resolution and filesize and contain the same information as one that is at 1,000,000,000 DPI. The reason you aren't getting a 'correct answer' is because your question is wrong. Youtube does not recommend or provide a "DPI" to use because the word you are looking for is resolution.

Now to answer your question if you replace 'DPI' with 'resolution', the resolution that youtube needs for channel banners is as such:

Recommended Banner Resolution is 2560 x 1440 pixels
Minimum image dimensions: 2048 x 1152 pixels, with an aspect ratio of 16:9
Minimum image safe area for text and logos: 1235 x 338 pixels
File size limit: 6MB

Look/search at the youtube help pages. There highly interesting and learnfull.
Also about what sound formats and such. About all kinds of specs.
Digging into it will save you problems/dissapointments later.

Also goes for the specs of other services.

Why? If you upload at their specs your video won't hardly be converted (aka altered, other then you wish) by them. If you yourself render/export to the recommended specs you keep that in your own control, and not let them do that (other then you want). Eventually will give you the best result at your wish.
We here (video, music, at many services) always upload at the best recommended specs the service demands. For exactly this reason.