If your screen is very bright you'll burn more battery - so turn on auto brightness (Settings - General - Accessibility - Display Accommodations), if you have it running at max, or otherwise set it to a consistently low percentage.
While you're there you may want to reduce visual effects like animation and 3D, in Accessibility.
3. Turn off Siri listening in on you all the time
Let's be honest, this is a bit weird and invasive, regardless.
Go to Settings - Siri & Search.
4. Turn off apps with high background activity
Going to Settings - General - Background App Refresh should reveal the apps that are sucking the most of your battery.
Give them a good ol' culling.
5. Turn off AssistiveTouch
Settings - General - Accessibility - AssistiveTouch.
This feature is designed for people who will have difficulty touching the screen.
If it doesn't apply to you it could be useful to dismiss it.
6. Low Power Mode
This is the hail mary option, the nuke-it-from-space-and-disable-a-lot-of-useful-features alternative.
It will prevent your phone from using data in the background at all and is found in the "Battery" section of settings - it's one step away from Airplane Mode, which is so barbarically restrictive we're not including it.
7. Buy a phone with a decent battery life
This has been Apple's weak spot for years.
If a good battery life is a key feature for you in a smartphone, there are better and less expensive options.