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
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.