You should absolutely keep some ketchup around the house. And soy sauce, and Sriracha, and a good BBQ sauce. And some honey. But if you really want to add flavor to all kinds of dishes (I don't mean fancy stuff, but like, if you want to punch up a sandwich or a salad or make some dinnertime staple seem more cuisine) then I have a suggestion of two key things you should keep around at all times: roasted garlic and pickled onions.
It's surprisingly easy to make these two things. (And technically, if you roast the garlic by submerging it in oil, you can technically call it a "confit" and sound very fancy and French.) Accordingly, I have made a brief YouTube video explaining how I do it, and it's a vertical video, to demonstrate that I'm fluent in the vernacular of the Kids These Days. Please enjoy, or delight in how I did this all completely wrong. Bon appétit!