You'll make a lot of decisions about whatever you are building. Both explicit and implicit.
When you make a decision, document it as close to the code as possible. Your current team, your future team, new developers and future you will all thank you for the context provided by answering "why the hell did they do it this way?"
Everything makes sense at the time, not everything makes sense in retrospect.
Architectural Decision Records save you from having to do digital archaeology or guess at why things are the way they are. There's great tools out there to help with this.