What YouTube actually reads from descriptions
Two parts of your description carry the heaviest SEO weight. (1) The first 150 characters, what shows above the 'show more' fold and what YouTube's algorithm weighs most. (2) The first paragraph (first ~250 chars total), secondary ranking signal.
Everything below that helps for viewer experience (timestamps, links, social) but barely moves rankings. Front-load your keyword + value in the first 150 chars. Don't open with 'Welcome to my channel!' or 'Like and subscribe!', those waste the ranking real estate.