What electronics are inside the LEGO Powered Up hub?
The bulk of the space inside the Powered Up hub is consumed by the housing for the 6 AAA batteries. A small portion of the top of the hub contains the main printed circuit board. Here's how it looks removed from the hub:



And here's another shot showing the back:



The main chip is very similar to the one used in Boost. It uses an STM32F030 from ST Microelectronics. This includes a 32 bit ARM Cortex-M0 clocked at 48Mhz.

For Bluetooth Low Energy (BLE) communication, the Powered Up hub uses a CC2640 from Texas Instruments.

The largest chip on the underside of the board is an LB1836. I haven't done any testing on the traces, but that part is a dual-channel stepper motor driver and is placed near the output ports, so it is almost certainly used to drive the output current to servo motors. The front side includes a pair of 74HC2G125DP chips near the output ports. These are likely used select which CPU pin the output pin is connected to.