Can I urge you to look through this forum at the many posts about this subject. There is a lot of experience on here and different ways to tackle the issues. It takes a lot of time to keep replying the same information, so please do some searching here on the forum. Those of us that have had the phone for a month or more have suffered many teething issues and some ways to resolve problems.
As a start, you could read my post from a couple of weeks ago, and which made massive differences to my own battery life.
@Nashy Helpful advice in the thread set up by @smiley . You may also wish to look at this article but not necessarily wish to follow all the recommendations. Not a fan of constant Dark Mode myself and think you need best combination of battery life and enjoyable experience with the technology (do not have the S21 myself mind you) https://www.samsung.com/nz/support/mobile-devices/11-tips-to-extend-and-improve-your-samsung-galaxy-...
I am aware that some problems appeared after February security update , apparently a fix in the pipeline. https://androidcommunity.com/galaxy-s21-battery-issues-appear-after-february-security-update-2021021...
I do not work for Samsung or make Samsung Products but provide independent advice and valuable contributions.
My Device- S21 Ultra 5G ( SM-998B/DS ) CSC= BTU , 12GB/256GB
One Ui 5.0,Android 13 .