Iphone SE(2020) 2nd generation ios 18.1 battery drain issue

FIRST: You should always charge overnight, every night, and enable automatic nightly backups. If you do this you should not need to charge during the day.

After an update there are housekeeping tasks that run in background, so for the first few days after an update battery energy higher than average will be required.

What do you see in Settings/Battery/Battery Health? What is the maximum capacity.

You have no choice about battery degradation. Batteries are consumables; they lose a little capacity every time they are discharged, then recharged. On average this works out to about a 1% loss for every 25 “full charge cycles”. As one example, if you charge the phone overnight, every night (and that is what you should do; it is a best practice), it starts the day at 100%. If it drops to 20% by the end of the day before you charge it again overnight that counts as 0.8 full charge cycles (20% to 100%), or about 24 full charge cycles per month of use. For this example your battery capacity will lose about 1% per month. Of course, if the end-of-day level is higher than 20% the capacity loss will be a little less, and if it is lower than 20%, or you charge it during the day, the capacity loss will be higher.