Phones are never really inactive. While most of the subsystems draw very little power on stand by, they still need to wake up occasionally to check on the world. There are the Bluetooth, WiFi and phone radios that will need to ping out and stay active. Radios still take quite a bit of power and may use more depending on signal strength and distance to the routers or cell antennas.

There are multiple system timers that manage things like updates and other functions. Those still take a little bit of power. Email is still going to get checked and text messages need to be updated

Browsers are a huge power suck regardless. While I can't say for certain, it may be allowing any open tabs to periodically refresh in the background. For the most part, phones do a really good job of halting applications that are in the background that aren't considered "core" services. Some apps might be noiser than others, but it's not nearly as bad as it was many years ago. "Task killer apps" are basically useless and have been for a long time.

Also, there is just your average battery lifespan and it's natural tendency to get shorter with every charge.

You would need to turn on any power saving features to improve battery life. TBH, after 18 hours, still having a 25% charge is really good.