It’s good practice to keep the oil topped up. When oil light comes on then it’s last chance saloon really.
On the dipstick there is usually a min mark and a max mark. Usually (but not always) the difference between min and max is 1 litre, most petrol stations sell the 1 litre top up size bottle.
Where does it go? Oil gets burnt away with the petrol but only in tiny amounts but over time it adds up and shows this on the dipstick. It’s normal to top up oil occasionally.
Go on euro car parts website, enter your reg number and the page will show every part they sell for your car. Go on service items/engine oil and it’ll show you a range of oils sutiable for your car. Petrol station oil is vastly overpriced, euro car parts is about the cheapest you’ll get.