It's largely the background data-hungry apps and the obsessive screen time with foreground apps. As someone who almost uses their phone like a dumb phone, I charge my Pixel 5a about once every 7 days now, when it gets to around 20% remaining. It was more like 10+ days when I first got it, but that was also before I decided that avoiding the 0-20% range might be better for long-term battery health.

The most power-hungry apps I run are Slack, the built-in GMail and Messages clients, the Garmin app to periodically sync with my watch, and sometimes Firefox if I actually spend time browsing there instead of on my preferred laptop environment. The camera app also eats power when actually taking pictures or video, but I guess I rarely do.

When I first start using a phone, I go through a little effort to disable things I don't want via the system apps menu. For me, that includes the Google Assistant and their native launcher, because the last thing I actually want is to trigger search functions willy-nilly. If I want to search, I'll open Firefox and search...