Titles: 100 characters, but fewer are seen
YouTube lets a title run to 100 characters, and the counter tells you when you pass that. What viewers actually see is shorter: search results, the home feed and suggested videos cut long titles off, and how much shows depends on the screen.
Write the title so the first few words carry it: the topic and the reason to click. Anything after that is extra context for people who expand it, and for search.
Descriptions: 5,000 characters, mostly below the fold
5,000 characters is a lot of room, and most of it sits behind "more". The top two or three lines are what people read under the video, so start with a plain sentence about what the video covers, then the one link you most want clicked.
Below that, use the space for chapters (timestamps starting at 0:00), links, credits and the questions people ask in comments. Chapters and links count toward the limit like any other text, so a long chapter list on a long video is the usual reason a description gets close to it.
Shorts and the channel page
Shorts use the same limits as other videos: 100 for the title and 5,000 for the description. Viewers in the Shorts player see very little of either, so the title matters more than the description there.
The channel description is separate, capped at 1,000 characters, and shows on your channel page. Your handle is 3 to 30 characters of letters, numbers, periods, underscores and hyphens.