Direct means the visit arrived unlabelled.
Google Analytics sorts every visit by where it came from, using two clues: the referrer (the site that sent the visitor, as reported by their browser) and any campaign tags in the link. A visit with neither is filed as (direct) / (none).
Direct is meant for people who typed your address. In practice it also collects every visit that lost its label on the way.
The YouTube app often drops the referrer.
When a viewer taps a description link in the YouTube app, the page opens in a browser that often has no referrer to pass on. With no tags on the link either, the visit lands in direct.
youtube.com never names the video.
Clicks from YouTube in a desktop browser usually do carry a referrer, and Google Analytics shows them as youtube.com, grouped under Organic Video. But browsers pass along only the site, not the exact page, so the referrer says youtube.com and nothing about the video.
Even your best-labelled YouTube traffic is one bucket for the whole channel.
Switching devices breaks the trail.
Google Analytics recognises a returning visitor by their browser. A buyer who watches on their phone and buys later on a laptop shows up on the laptop as a new visitor, with no referrer and no tags. The sale lands in direct, with nothing pointing back to the video.
Label the visits with UTM tags.
Tag every description link with UTM parameters. The tags travel inside the link, so the visit is labelled even when the referrer is missing. How to add UTM tags to your YouTube links has a naming scheme that names each video.
To check what's arriving today, open Google Analytics and go to Reports, then Acquisition, then Traffic acquisition. Switch the table to Session source / medium. Tagged YouTube visits show up as youtube / video, untagged ones as youtube.com / referral or (direct) / (none).
Connect the sale to the video.
Tags label the visit. To see which video sold, the purchase has to be recorded against that visit too. How to see which YouTube videos sell your course walks through the ways to do it.