How did the last update kill my battery?

Before looking at causes, ensure you turn off charge limit, turn on Optimised Charging, and charge it overnight, every night to 100%

Causes of drain can be:

1.    First all there will be a lot of housekeeping, re-indexing and photo curating going on after any update or after a transfer from another phone.

2.    If you have iCloud photos turned on there will be a lot of Wi-Fi traffic while they are re-curated. iCloud photos is turned on by default .

3.    There may be apps that are inherently incompatible with the new iOS. Start by removing any non-mandated VPNs or security apps. They can be a strong contender for runaway power usage redirecting your web traffic.

4.    Animated lock screens such as weather or astronomy, and widgets requiring location all use a lot of battery. Avoid if possible.

5.    Many mainstream apps need an urgent update, so go to App Store and scan through your apps for those that need updating. While there, offload any apps you haven't used for some time. They are unnecessary update burden. 

6.    Is it summer time? Outdoors your phone will automatically turn up the brightness to maximum. It will also get hot in the sun. Consider using dark mode on OLED phones.

7.    Check the battery usage chart in battery settings for any obvious bad apps.

8.    Any streaming apps or apps that can run with phone closed, or Clocks that hold the phone unlocked, always force close them afterwards, despite the accepted wisdom not to.

9.    Always on will obviously use more battery than without it.

10 Check battery capacity is above 80% in Battery health

11.Settings, Cellular (or Mobile data) turn off as many functions as possible as cellular uses more power than Wi-Fi

12.If you sync by computer then make sure that"show this device when on wifi" is turned off. Or if you need it on, then ensure "automatically sync" is turned off. 

13.Hi-Res Lossless Apple Music is a heavy power and data user, reserve it for your home streaming device.

14.Push notifications for heavy email users could be replaced by 15 minute fetch.

 See also the Apple article

If the battery in your iPhone drains too quickly - Apple Support

 Sick Battery

Even batteries with "Health" greater than 80% can be sick. Symptoms are

• Battery charge level % jumps, gets stuck, or drops to nothing suddenly
• Battery seems to have a steep discharge curve overnight even after doing all of the above checks and after waiting a week after tge update.

In these cases see another User Tip Check iphone battery for self discharge

Official guide If the battery in your iPhone or iPad drains too quickly - Apple Support