I think the title is less of a description of the video’s content than it is clickbait, but fair enough. From my perspective, if you know a few variations for 4-4 approaches, 3-4 approaches, 3-3 invasion you should be okay. Generally there are only 3 responses:
- extend (with n-space jump, keima or ogeima)
- attach (on top, under, side) or
- pincer (n-space, high/low).
Naively calculated, that makes for about 2x2x11=44 variations starting from 3-4 and 4-4 plus maybe 3 sansan variations you should know, leaving subvariations and transpositions out of consideration for now. You might therefore want to look for variations that quickly settle things, as they will be easier to remember. Another somewhat advanced topic would be double approaches, where you leave your cornerstone alone for a while and it is then approached by 2 of your opponent’s stones. This adds another, say, 10-20 main variations for 4-4 and 3-4.
What joseki generally have in common is that players end up with either solid shape (no severe cuts), light shape (you can abandon the shape and if necessary sacrifice a few stones to jump out, retain influence or create/keep aji for invasions later, etc), or just a live group, mostly at the expense of outward thickness.
When to use which is obviously a matter of strategy.