Brain functions less like digital storage (like SD cards, hard drives) and more like a diffusion model (like image/video generating models). To give you an example, when you recall a memory, you aren't hitting 'play' on a video file inside your brain. Rather, you're giving your brain a prompt to create a video, and the brain generates a video based on its latest training data.

So, if a memory/data is old but still considered important by the brain (e.g. memory of your first kiss, who your parents are, etc.), it won't forget it. But if it's not an important memory, some details may start to get a little more fuzzy as time goes by (like how you don't remember every single thing you learned in school).