Look for key points in the battery life with significant drops along with high activity, click that area and see what was going on.
Here is mine for example....
At that particular drop the phone (me calling someone) used the most amount of battery level.
Keep in mind that "battery usage by app" is a PERCENTAGE equally 100% used in that time period and is not indicative of the app actually using a lot of battery.
For example...
Siri was 100% of the battery consumption between 3pm and 4pm. However this is only because its a percentage based on 1 thing, 1 of 1 = 100%. And if I look back up to the battery level I can tell that it dropped maybe 1%. So Siri used 100% of maybe 1% over the course of an hour aka not a lot.
You have an iPhone 8 Plus with 100 battery health? Did when and where did you get the phone? Potentially 3 year old phone with 100% battery seems hard to believe. Not saying you are lying but an aftermarket battery reporting max capacity isn't uncommon if the phone was bought 3rd party....