It’s called the Second Law of Thermodynamics. Anything that creates or uses energy does so with some loss. That lost energy is expressed as heat. So when you charge the phone it generates heat in the power source, heat in the charger circuit in the phone, and heat as the energy goes into the battery. The faster the charging, the more heat is generated. Likewise when you discharge the battery; not all of the energy from the battery gets to the circuits that use it. Some of it becomes heat. In particular, fast charging with a 20 watt or higher power source will cause any smartphone to get very warm, so warm it may have to pause charging to cool down after 30 minutes or so.
If you use the cellular network for voice or data, converting energy to radio frequency signals is very wasteful; only about 30% of the energy that goes into the network components comes out as radio signals, the other 70% becomes heat. And signal strength matters; a 1 bar signal requires that the phone boost its transmitter power to maintain a connection, and it isn’t linear. 1 bar uses 10 times as much energy as 4 bars. You didn’t mention your phone model, but an iPhone 12 or newer (including SE 3rd edition) with a 5G signal uses about twice as much energy as an LTE signal to maintain a connection.
And some apps use a lot of energy, and thus create more heat. Examples are streaming video or audio, or interactive video games. Also apps like Facebook, Instagram, TikTok, Snapchat that use video streaming.
Next, can you quantify “draining so fast”? If you charge overnight, every night, with either 80% limit or, better yet, Optimized Charging, the phone should not need charging during the day unless you spend hours playing interactive video games or streaming video. If you don’t have to charge during the day the phone is working as designed.
