Focusing on the wrong things
Imagine that you have been working as a frontend engineer for a long time and now you want to move to backend. There’s this mix of anxiety and excitement about learning new things. The anxiety, at least to me, is usually around productivity. As a newcomer to a field, you want to be up-to-speed as soon as possible. However, people tend to focus on the wrong things. For example knowing a new language in detail, or some piece of infrastructure (e.g. AWS). Instead, what I recommend is to do pair programming with people familiarised with the system. Try to have at least 1 to 2 hours per day until you feel productive. See these sessions like having a personal trainer. It’s unlikely that the new language, or the new tech, is so alien that you feel like a junior that never coded. So the focus should be instead on the workflow. Pay close attention to tricks and setup that make your mentor productive. Even if you are not confident about the quality of what you are producing, these can be remediated at review time (e.g. you can ask people to be more thorough on your PRs).
Deep knowledge about technology will be of value, but initially the primary focus is getting stuff done.