What choice can I have at work after learning plc and frequency converter by myself?

PLC and frequency converter are mainly used for automatic control, and it will be useful if you learn it well.

Plc and frequency converter are widely used now. When you teach yourself, you should not only know their basic structure, but also understand their working principle. In this way, you can do a lot in many places. I used to do after-sales work in a machinery company, but I had to change careers because I didn't understand this. Besides, even if you don't enter the factory, you can open a special repair shop if you learn it well.

Good luck ~