When you first start photography, it's pretty easy to get the exposure right by letting the camera do most of the heavy lifting. However, cameras are staggeringly stupid, even in today's age of AI-infused-everything. If you want to achieve a certain look, or you want to protect the highlights, or you want to expose for the shadows, you need to know how to use some tools your camera has.
When I first started photography, one area I couldn't get my head around was avoiding a perfectly balanced scene. When the camera tries to balance out everything, it can result in an incredibly dull image, which happened time and time again for me. It wasn't until I first went into manual mode and under-exposed a stormy sunset that I realized the camera could capture exactly what I wanted, it just needed me to instruct it on how. The best place to start with how to instruct it is to understand the different metering modes.
What were your beginner mistakes with exposure?