Why does my phone's battery drain so fast and get hot when the 'Hotspot' is constantly on?

It's because your phone is performing two sets of high-power 'signal conversion and data forwarding' simultaneously! Normally, when you use your phone, it only needs to receive data through the mobile network (4G/5G) and display it on the screen. However, when you turn on the hotspot, your phone instantly becomes a 'miniature wireless router.' It has to pull in the mobile signal at high power using its mobile communication module (baseband chip) and, at the same time, immediately package the data and broadcast it at high frequency via the Wi-Fi/Bluetooth chip. This dual, high-load data computation and signal modulation keeps the processor and communication chips at maximum capacity for extended periods. The 'Joule heating' generated by the high flow of electrons accumulates rapidly, which is why your phone gets intensely hot and the battery drains dramatically!