I also built a simple oscilloscope with the help of my teacher.

Recently retired, I built a simple oscilloscope by following my teacher's project. I simply replaced the PCB with surface-mount components; everything else was copied from the project. I learned all the lessons step-by-step from the video tutorials on an open-source platform.
This is the soldered PCB; everything except the interface connectors is now surface-mount.
This is the assembled PCB. I added a programmable interface for direct serial programming.
 
I wrote the program and tested its own signal generator. I tested
a 10kHz sine wave and
a 10kHz square wave.
The schematic wasn't modified; it shows a GD32E230, but the minimum system is the same for both STM and GD, only the program is different.
It's a small success, and JLCPCB is a great place to learn and improve. There were a few minor hiccups during the process. I used a heated soldering station, which resulted in some cold solder joints. During testing, there was only an interface without a waveform. I troubleshooted step-by-step from the input to the ADC, and once I solved the problem, the waveform appeared.