Often it is as you assumed. You take what you need and put the lid back on. What happens after the can is opened depends on the product. Salt may draw water, fat may go rancid and coffee tends to loose flavour. That would shorten the shelf life. Often there is some fine print on the can that states a timeframe for consuming the product after openening the can. You may have to read the info on the can to find out.
There are products that have a small number of servings in separate packages. Most common around here are mashed potatoes (3 x 4 servings). They may not be freeze dried and come in cartons though.