Running the battery down to zero regularly will kill it. If it accidentally reaches zero charge it immediately (within hours) or it will die completely. Zero is not really fully discharged; Apple “lies” and says it's zero when there is still some charge left in it because a lithium battery that actually fully discharges will be damaged, will lose capacity, and after a short time will be unrecoverable and need to be replaced.
Thus, it may be too late. But go to Settings/Battery/Battery Health and see what message is displayed.
The absolute best way to get maximum use on a charge, as well as slow the decline of battery capacity long term is to enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight, every night. The battery will fast charge to 80%, then pause. During the nighttime pause the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern. If you enable iCloud Backup (Settings/[your name]/iCloud - iCloud Backup) the phone will back up overnight also, assuring that you can never lose more than the current day’s updates. Here's more information→About Optimized Battery Charging on your iPhone - Apple Support
