Pre-pandemic, I used to regularly host dinner parties. I love hosting them; my friends basically never do (not their style). The style of each gathering can vary - sometimes I cook/provide every dish and friends can bring something only if they feel inclined which I usually politely refuse (most common), sometimes it's potluck and I provide the main course since I'm hosting, and sometimes it's delivery/takeout, just at my home (each person looks at the menu beforehand and tells me exactly what they want). The only time I'd ever charge anyone is in the last scenario, and I make it very clear with everyone beforehand (e.g., "Let's all have dinner at my place soon, we can order food from [specific restaurant] and you can all just Venmo me later"). I do this pretty rarely anyway, as it's easier and more satisfying to just go to the restaurant itself.

Your friend charging you and not making it explicitly clear beforehand, for a relatively cheap meal she took the majority of leftovers for, is pretty tacky, and not good etiquette at all. Unless explicitly stated, it's generally expected the host covers all costs, and you can offer extras as a guest out of politeness, but by are no means obligated to bring anything other than your own company. I would definitely not go to a friend's gathering if she did this to me again - the money isn't the issue per se, it's the principle of her springing it on you and putting you in an uncomfortable situation (e.g., what if you couldn't afford it?). She also seems to have made a profit off this meal based on your other comment, which actually made me cringe. Overall, your impression of everything is very justified, OP!