I hear you. I also disagree with the argument that a plant-based diet is somehow invalid because it requires going out of your way to get enough B12.
I’m a big advocate for greater awareness about B12 both in vegans and omnivores and have written about this here.
I think it’s important to clarify that we (all animals) rely on bacteria to make B12 for them.
Ruminants (cows, sheep) do very well because they house B12-making bacteria in their rumen (specialized gut). B12 synthesis depends on the element cobalt, so some cattle need to be fortified with B12 if they are grazing low-cobalt soil.
Other animals get B12 either by eating faeces (rich in microbes) or flesh of other animals who “used” microbes to make it. These days, chickens often receive Vitamin B12 supplements. You can read more about supplementation practices here (note that the info is from a supplement maker so bear in mind potential bias to suggest that more is better).