The taxpayer decides how they want to deal with it and it also depends on when the contribution was made. For instance, it can be withdrawn; or, if contributed in 2021 for tax year 2020, then apply the overage to 2021, assuming they will qualify to contribute in 2021.

Search the web, as all the investment brokerages maintain "how to fix excess contribution" instructions. I also like investopedia guidance:

https://www.investopedia.com/articles/retirement/04/042804.asp

And use the IRS resources such as tax topics and pubs:

https://www.irs.gov/publications/p590a