Is the damage an actual hole (as if something chewed into the fruit, where the surrounding tissue is still its normal color and firm), or is it a sometimes-sunken, rot-like darkening? While hornworm caterpillars can chew into fruits as well as consuming leaves, usually their damage is distinguishable from the very common disorder called Blossom End Rot. Birds can also peck into tomato fruits if they are thirsty, but we don't tend to see this on the bottom of fruits, just the tops or sides. You are welcome to share photos of the damage if you would like our impression of which issue might be the cause.

For hornworms, the best solution is to just pick them off the plant and toss them somewhere birds or other animals can snack on them, though finding them when they are well-camouflaged can admittedly be very challenging. (Try looking after dark with a flashlight, when they might be feeding more openly and hiding less.) For Blossom End Rot, you can't salvage the individual fruits with damage, but you can use the management measures in the linked page to reduce the risk of recurrence.

Miri

Thank you for the photo. Although the image is not quite in focus, the hole does not look like Blossom End Rot or physical damage from a caterpillar or bird. Instead, it resembles "catfacing," a common type of deformation that tends to occur on earlier-season crops back when the weather was cooler. University of Massachusetts points out that other causes of this issue are not completely known, though might still be related to temperature extremes or fluctuations during the growing season. Alabama Extension adds that it should not affect tomato flavor for fruits affected by the disorder. Both linked pages include tips for discouraging future catfacing on ripening fruits, though since the phenomenon is not fully understood yet, it's hard to say which approach will be the most successful, and some tomato cultivars may be more prone to developing this than others.

Miri