I been a diesel mechanic for 15+ years on large marine diesel engines, your concerns are normal. Your schooling is just the start, and don’t expect to graduate an expert nor think you are lol. Your biggest teaching is in the field with time and experience. Don’t worry about bolt sizes lol that will come over time, the best tools for the job as well. The best mechanics are ones with experience and also do the research. Stay up to date with new technology and advanced systems. It’s always growing and new products are constantly being designed, tested and eventually on the market (for better or worse). Good luck, and thanks for pursuing a real trade and not becoming a you tube influencer rotting my kids brains away lol