Can someone explain me please?

Mikasa and jean got a kid together. The kid at the End is their kid.

I don’t think the kid at the end is their kid since mikasa died of old age meaning her kids must be 40-60. I think it’s just a random kid who might start things over again if it’s continued which is probably not

About the other people and the chair that I thought Levi was using?

Levi would of been dead since his age difference with mikasa and Jean is big by almost 20 years