I've heard several approaches to endscreens.
1 Some will leave endscreen selections to what YouTube deems as the best, I'm not sure how detailed it's selections are, but one would assume that based on the seo you put into the video (including the audio) it would select the best video based on your gallery of what performed well in a similar style or topic (based on seo metadata and audio (it scans audio for auto captions, it scans for copyright music, it scans for swearing, so no doubt it also scans for metadata information in what you say))
I haven't trusted auto selection much because...
2, some try and push traffic to a video that might not have performed as well as thought. Theoretically this might not be advised because there's usually a reason why the video didn't perform, i.e a topic the audience doesn't care about, the thumbnail or title stinks, or the video it'self got such poor audience retention that YouTube don't see it wise to push, and if YouTube don't push it, why waste potential watch time pushing to a poorly performing video when you can...
3. Push to a GREAT performing video. If a poor performing video is selected, it might end watch session times, pushing to a great performing video that still is relevant, can potentially keep the watch time session going further, getting even more views, helping boost it's suggested impressions long term.
I've typically gone with either
2 related specifically chosen videos related to the current video
Mmix between 1 specific video and 1 playlist, to give the audience options.
Or I'll select 1 specific video ONLY, if it's a continuation of a series, or very specifically related to the video. And I've heard people say to stick with one video rather than 2, because 1 sets up a direct choice of where to go next.
I used to have 20 second end cards, but found having a 20 second end card could be too long, you want to get people moving right away, not potentally cutting of with 20 seconds left to go. So now I shink it right down to 10 seconds, keep it short and sweet. Remember, if things go well, not only do you have the end card to go with, but YouTube will also recommend more of your videos after the fact, in a list of 6 or so auto selected videos to play up next, and if the viewer has auto play enabled, it will automatically go to the next video (and hopefully it's one of your own videos and not a competitor's video)
My end call to action is usually "If you've made it this far and haven't subscribed already, you should probably do that now, or simply watch this next video". Generally my videos tend to be long, so if someone's made it to the end card at all, that's about 10% of people, so I can make a personal video to send them to next and they probably will listen to the suggestion.