STC51 is a single chip computer with MCS-51 as the core launched by STC company. It is basically the same as AT89C51, but it can be burned directly through the serial port, so it is widely used.
MCS-51 is an entry-level, classic MCU, characterized by simplicity, so it is widely used in teaching. But because it was a chip from the 1970s, the design and resources are now seriously inadequate.
The STM32 is based on ARM's latest generation cortex-M core chip, introduced by ST, which is widely adopted because of its ultra-high cost performance and simple library programming.
The STM32 series integrates almost all functional modules in all industrial control fields, including USB, network, SD card, AD, DA and so on. The main frequency is 72M.