I assume the YT algorithm determines for each viewer when they generally click away from the type of video they are currently watching, and would place the featured video link some time before the click-away point for that viewer.
For example, say I watch all Thomas trains videos for 4 minutes, and all Play Doh videos for 2 minutes (on average).
So in the Thomas video I'm currently watching, it would place the featured video link sometime before the 4 minute point.
For a Play Doh video it would place it just before the 2 minute mark.
This way the algorithm ensures you watch as much of the current video as possible, and when you historically click away, it offers you another choice from the same channel to keep your session time up for that channel.
That's just my understanding, could be wrong.