The reason why some people recommend putting Android phones in aeroplane mode or to shut them down completely is so that the battery can draw as much current as possible and charge itself fast.
When the charger is plugged in, the Android system sends a broadcast (a system-wide message that any app can listen to) with the action
android.intent.action.ACTION_POWER_CONNECTED
. Several apps will listen for this broadcast, and as soon as it is received, they will activate their routines and start some battery-intensive work. For background work, we use Android
WorkManager, which has an option to start a work when the charger is connected. Therefore, in most phones with a number of heavy apps, when you plug in the charger, a part of the charger current will be used behind the system rather than charging, and charging becomes slow.
Most of the apps that activate when the charger is connected are doing work over the internet. Engaging aeroplane mode disconnects internet, and those apps are paused due to lack of connectivity. I haven't tried, but probably just turning off mobile data and WiFi will probably have the same effect as aeroplane mode. And aeroplane mode with WiFi connected will probably defeat the purpose.
If you shut down your device, you are making sure that all the current delivered by the charger can be utilized to charge the battery.
But aeroplane mode does not make a difference if your phone is switched off. Switching off means it's completely off. So, all cell services are suspended automatically.
Shutting down is probably more effective than aeroplane mode alone because aeroplane mode still allows apps to run, and that will consume some battery. But the number of apps that do not require internet access at all is steadily decreasing, so maybe after some time, aeroplane mode will suffice. It also depends on how well the app is written. Honestly, without data, it is difficult to judge which is more effective.
Personally, I never turn off my devices/engage aeroplane mode while charging. I just don't like it, as I am not sure how effective those methods actually are due to lack of proper statistical data. If your device supports fast charging, then there is no need to do any of these.