I had an iPad that I bought 5? years ago and I could leave that thing in sleep mode for literal weeks at a time and I'd still have a mostly full charge.
I "upgraded" to a Pro 2 years back and the first thing I noticed was it'd go from a 100% charge to 60% to even 40% overnight. All apps closed, freshly rebooted and did nothing but charge it the previous night before unplugging it.
A lot of comments online say "This doesn't happen. You're doing something. And it's certainly not wifi and bluetooth because those don't use battery while idle so do NOT turn those off!" I tried everything that was recommended, turning off iCloud, uninstalling random apps, etc.
Then I got into a habit of turning bluetooth off every night and my battery would only drain about 4-6% a day while idle. Turning off wifi as well and I get about 2-3% drain per night. I have no clue what Apple is doing, but when putting my iPad to sleep at night with any sort of wireless on, the device connects to every fucking thing it can find but wouldn't do while awake. I never even synced my Airpods with my iPad and made sure to delete them when they did connect, but still, if I leave bluetooth on before going to bed, my iPad will connect to my airpods while they're in the case, drain them to 0% (even after fully charging the Airpods before sleeping. It's not even possible to drain the case's battery with actual usage that fast!), then drain 40% of its own battery.
I can't make sense of it.
In short, turn all wireless off when you're done for the day. iPads/iPhones are annoying in that you can't turn them off from the drop down menu though, so you need to go through settings or make a shortcut.