What belongs in the first two lines
YouTube only shows the first one to two lines of a description before a viewer has to click "more," and search engines weigh that visible snippet more heavily too. Put your clearest, most specific sentence about what the video actually covers right at the top - not a generic channel intro - then let timestamps, links and hashtags fill out the rest below. The timestamps this tool generates are evenly spaced placeholders; swap them for the real times once the video is edited so chapters line up correctly.
Why the generated timestamps are evenly spaced placeholders rather than left blank
Having the chapter structure already in place - correctly formatted, in ascending order, starting at 0:00 - means you only need to adjust the actual numbers to match your final edit, rather than building the whole timestamp block from scratch afterward. The placeholder spacing exists purely to get that structure right early, not as a guess at your video's real pacing.
Why links and hashtags are placed near the bottom rather than at the top
The most valuable space in a description - the part visible before a viewer clicks "more" - is reserved for the one or two sentences that actually describe the video, since that's also the part search engines weigh most heavily. Links and hashtags matter, but they're secondary information that a viewer who's already engaged will scroll to find, which is why they're placed further down rather than competing for that limited top-of-description space.
Why hashtags are capped in how many visibly matter, not in how many you can add
YouTube only displays the first three hashtags prominently above the video title, though any additional ones you add still function for search purposes - understanding that distinction matters for deciding how many hashtags are actually worth the effort of adding versus how many will mostly go unseen in that prominent spot.