Thanks Emily,
Not too sure whether 2021 or 2018 has much bearing over it - crxp is crxp whatever its age.
This is a very difficult one to confidently advise on, but having said that, you have what appears to be an incorrect spec. low grade battery, with no evidence that it was ever coded to the car when replaced. You now have a dodgy alternator ( as a result?) and that may (in caps) be affecting the correct operation of many of your electrical systems.
On the one hand, you may find fitting a branded new battery fitted to the car and getting it coded, along with rechecking the alternator consistent output, and all will be well - but no guarantee that it will. I will however be more confident in claiming that if you don’t to that, you are more likely not to fix it if you get anything else checked and worked on.
Kind regards,
Gareth.