Covid Variants: where do they come from?

Why do new variants keep popping up? Some people are asserting that vaccination is the cause of variants. What does the best evidence today say about that?

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8313503/

https://www.nejm.org/doi/full/10.1056/NEJMc2031364

https://www.nature.com/articles/s41586-021-03291-y

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8301427/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8348317/

https://www.medrxiv.org/content/10.1101/2021.05.31.21257591v1.full.pdf ... https://www.youtube.com/watch?v=kLDqGRsCo6U