There isn’t one answer that works for all.
Iʻll admit being lucky I found this profession. I wasnʻt looking for it. I was looking for a job as I was approaching a time of crisis in my life.
There was no “technical communications” coursework for me, no certificates, no apprenticeship. I would not recommend this to others, although it worked for me.
Remember that I had basic written communication skills before my first gig. Since then, I keep learning about my profession. Things change. Never stop learning.