I have stunning battery life on iPhone 15 Pro and 15 Pro Max. There are a LOT of factors which affect battery life.
Cellular for example. Do you have your phone set on 5G or 5G Auto? If you have it set on 5G, it will work very hard to maintain a connection to a 5G network, especially if the 5G signal is poor. That takes a lot of battery.
Location Services take a lot of battery. Do you have location service on for every app you can use location services.
You may have a rogue app, running in the background (they should suspend when not in active use), but I have seen apps continue to run. Check your battery usage statistics to see if an app is using a lot battery and make adjustments as necessary.
Then there are the things you may be doing. Do you stream music, play video games, watch videos, make a lot of phone calls? All those are heavy users of battery.
Also, after you set up your new phone, it's often a good idea to Force Restart it --> Force restart iPhone - Apple Support
And depending on how long it's been since you got your phone and transferred your data to it, it can take a few days for the data to be fully indexed, which also takes a lot of battery usage.