I have been learning English as it is mandatory in German schools since Grade 2 (7 Years old). I didn't have any interest in it and was generally bad until I started consuming English media around age 10 (Games, Movies, Music, Coding/Programming).
Because I always consumed this media and had friends with whom I could only communicate in English, I never really studied for English class.
Now I have a C1.2 Certificate but that is just reading/listening and a bit of writing/grammar. I noticed more and more that I am not that good with forming own sentences and coming up with vocabulary.
While I understand even the most technical and niche texts, and am pretty confident I can read everything (getting words I don't know by context), I couldn't even dream to write or let alone talk like that myself.
What is a good method to expand my available active vocabulary and how do I learn to actually use it? (Preferably by acquisition not really learning, because that is how I get languages best. Through using them, not studying).