What Has Thomas Müller Been Up To Since Ending His Soccer Career? Muller Profile

Can anyone explain me in a simple way? Today my friend asked me if you can use has instead of have here. Since there is no other.

Thomas Muller to leave Bayern Munich after 25 years CGTN

There are 200 individuals who each have two alleles at single nucleotide site of interest. I have read a similar question here but that one talks about the usage of has/have with reference to anyone. Here, i wish to ask a.

I am wondering if the following sentence is correct.

I'm not sure how to explain the grammar simply. Can anyone tell me where we have to use has and where we have to use have?