I want to know how the tuna can be kept for so long in a can. Do they use preservatives? Or is it just tuna and water (or oil or brine)?
The preserving effect of canning is based on
- removing all bacteria and fungi (normally present at least to some extent even in perfectly safe food) by a combination of heat and pressure over a certain time
- preventing new bacteria or fungi from reaching the food by sealing the containers
- avoiding oxidation by sealing the cans
- sometimes supporting this by ingredients that are very inhospital for bacteria and fungi like acids (pickles, anyone?) or sugar (less available water).
For your tuna, this means the heat/pressure of the canning process is sufficient to prevent spoilage for a long time.
Special preservatives are typically not necessary. Checking the label should confirm this. If one brand should have choosen to add them nonetheless, you can pick another manufacturer.