How do allergy medicines work?

I got thinking about this the other day after i accidentally ate something with peanuts in it and had to have some benadryl to stop me from having a bad reaction. yay allergies. So i thought, i’ve been taking benadryl my whole life and it always cures me, but i really have no clue how it works.I mean my body for some reason thinks peanuts are bad, what is it in benadryl that tells my body “nahhhh its all cool”

How do preventative allergy medicines work too? (claritin, allegra ect…) Why are we able to make preventative allergy medicines for some allergens, but not others? What is different about them?