Must I do a factory reset to easily cleanse my computer of every little nook and cranny download mod and other oddity I've accumulated

First time laptop, long time moron. I feel like I've done something horribly wrong, and I cannot figure it out. I've gotten games through Steam, I've gotten games through GOG. I've downloaded and uninstalled to buy and play others at other times. My Hard Drive is a mere 512gb and there's like nothing on it, and it's chock full. I can't figure out what to look into to manually delete files to make space for a recent summer sale purchase. And it's saying everything is full, and all I have is like 2 games that I can see. So what's the other storage holding? Like every save file and setting from the uninstalled games(?) I don't get it. I'm not savvy enough that even a simple explanation will work. And when I ask my kids they act dumber than I to avoid having to help 😔 If anyone can give me a little basic step by step that I can follow to get in and clean out my gutters, you'd be a hero It's to the point that I feel like wiping the whole thing, and cherry picking what I want to play to keep space free (because clean slate or not, I'm still a Stupids) and then I'd still be in the predicament I suppose eventually again. But I'm more concerned with the now, friends.. idk if this counts.. it doesn't seem to violate your rules. I don't know where else to go *tech support is literally what I need. I hope the moderator doesn't throw this out. I'm likely going to go to sleep and then work 12 hours tomorrow so I likely won't see any responses until a few days if you need feedback, though I don't know how to better describe 🥲🗽