Here are a few STEM focused ones I'm in. Note, that much of the content (due to the younger age demographic) is people asking for homework help and such a lot of the time. Funny enough, that has died down now with the advent of ChatGPT. Equally, I've met many field experts who hang out in these groups too and people I've eventually become friends and colleagues with:

Furlough (Great Startup Community): https://discord.gg/furlough

Science and Math Association: https://discord.gg/Yqf9GGjV

Mathematics: https://discord.gg/zZ94XYkt37

Control Engineering: https://discord.gg/mPwkRJKG

DIY Tech: https://discord.gg/diytech

Computer Vision: https://discord.gg/4BXczxNJwK

Adafruit: https://discord.gg/adafruit

Better C++: https://discord.gg/e3aXc2xUWr

Python: https://discord.gg/python

Unity Discord: https://discord.gg/unity

The Programmers Hangout: https://discord.gg/programming

The Coding Den: https://discord.gg/code

Game Dev Network: https://discord.gg/gdn

Developer-Ecosystem: https://discord.gg/deveco

OpenAI: https://discord.gg/openai

Makers HQ: https://discord.gg/QWtd5Z6N