Write While You Work
When getting into a new feature or bug fix or project, start writing things down. Maybe this is a physical notebook or just a Markdown file but just have some place to keep notes for yourself. This is different than any notes or comments you might share on the ticket or project doc discussions. These should be for you and make them as organized or ugly as you want.
The idea is to just have a place to keep everything you know in one place. Maybe this is writing down the flow of method calls for a certain piece of functionality. Maybe it's a place to keep some VERY quick and dirty scripts to add new test data. Maybe it's just notes from a conversation you had with a coworker that provided some clarity about the system.
In addition to the immediate benefit of just writing down what you know about the project so you don't have to keep everything in your head, these notes often become the basis for some new documentation that (once polished up a bit) can be a huge help to other people.