Too Long; Didn't Read
As a junior developer you are not going to know everything, and you are going to make mistakes. You have to learn the current tech stack just to be useful, but you also have to start learning the changes to that stack. Good companies will assign you a 'buddy' to help you figure things out when you are looking lost. There is a 99% chance that 99% of what you need that first exists exists. Don't reinvent the wheel because they need a specific type of wheel.